Ok, I'll start by saying that I'm not well versed in plumbing lingo so bear with me.

I have been forced to do a bathroom remodel in my house due to drywall installed behind my tub surround. As you can imagine, the wall practically fell into the tub about 6 months after I bought the place... figures.
Now, my house was built in the late 40's and had an addition put on in the early 60's and that is where the bathroom is.
Looking in the basement, I can see the front bathroom plumbing. I have a 4 inch sewer pipe running to the street and a 3 inch line for the toilet drain and another 3 inch line running through the roof and the drains for the tub and sink attach to this vent.
In the back bathroom, the plumbing is a bit more rigged... The tub has no vent and neither does the toilet. There is however a 3 inch pipe running vertically through the floor and roof from the main sewer line and the sink drain attaches to that. Now, I don't believe that simply venting the sewer line is "code" for a toilet and tub vent but I could be wrong.
Knowing all that, here is my question. I plan on replumbing all three fixtures. Can I remove the 3 inch vent pipe from the sewer line, cut that pipe in the attic, and run separate vents for the tub and sink, and then reconnect the two vents back to the 3 inch part going through the roof? The toilet has a vent pipe in place, but it is not connected to the toilet drain anymore.

Sorry if this is confusing, and I can take pictures if necessary. Thanks in advance!

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here is my question. I plan on replumbing all three fixtures. Can I remove the 3 inch vent pipe from the sewer line, cut that pipe in the attic, and run separate vents for the tub and sink, and then reconnect the two vents back to the 3 inch part going through the roof? The toilet has a vent pipe in place, but it is not connected to the toilet drain anymore

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That 3" sounds more like a drain or a stack then a vent. You can tie all the vents in the attic back to a single roof vent. This is called "reventing". But why all the extra vents from the tub and toilet. Aren't wet vents, (see image) allowed in your area? Back to you, Tom

Well it is my understanding that all fixtures need to be vented one way or the other. The 3" vertical stack literally comes through the middle of the bathroom floor. This bathroom is only 5' wide and 14' long so I wanted to remove that pipe and the wall that it is in to open up the room. But of course, since that pipe goes through the roof, I wanted to use that roof penetration.

The 3" pipe may have originally been used for something else, but as it is right now, it serves as a wet vent for the sink but that's it. The tub drain goes to the main sewer line before the vent, and the toilet goes to the main sewer line after the vent. I know that indiviually venting each fixture will require more work, but I'm not sure if there is a better way to do it and still keep the plumbing inside the wall.

Also, the tub and toilet are roughly 14' apart. Now, the toilet has a pipe behind it that also goes through the roof. Its about 1.5" but somebody in the past replaced about 3 feet of sewer line for the toilet with pvc and left that vent pipe disconnected in the basement.

In addition to the above advices: In my area, and under UPC, we vent every fixture individually. So if you want to install dedicated vents for each plumbing fixture than I see no problem here. You can join all vents in the attic and run it through the roof as one single vent.
That pipe behind toilet is most likely old vent that got never connected during remodel. You can utilize it if you wish. But first, test it to make sure it is not terminated somewhere in the ceiling.

Milos correct about his code. However if wet vents are allowed in your area you will save on both labor and material if you hook the way I've shown. You say:

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The 3" pipe may have originally been used for something else, but as it is right now, it serves as a wet vent for the sink

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Say What? %The sink IS the vernt from the stubout up to the roof, Anything below the floor is the wet vents. Check with your Building to see if wet vents are allowed. If they aren't then you'll have to go with Milos code. Good luck, Tom

So if I were to keep the vertical pipe, what size would the pipe need to be to properly vent a tub, sink, and possibly a toilet and still fit inside a 2x4 wall. I am assuming your suggestion would be to run the drains for these fixtures under the floor to a centrally located area?

Your picture is what has me confused, since it uses the 3" pipe and I want to take that pipe out completely except for the part that is in the attic.

I have attached a couple of rough pain drawings to illustrate what I want to do...

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So if I were to keep the vertical pipe, what size would the pipe need to be to properly vent a tub, sink, and possibly a toilet and Your vent would be 2".still fit inside a 2x4 wall.]

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Your vent would be 2" coming off the lavatory.
Your first picture won't pass because you're discharging a major fixture past a unvented minor one,
Milo would love the bottom one. You have installed a dedicated vent on every fixture. However, If wet vents are allowed in your area,( check with your local Building Department) all you need to do is connect the sink drain and let the lavatory vent the entire bathroom group. That way you would only have one vent instead of three. Good luck, Tom

So what you are saying is that if I run a 2" vent at the sink that I wouldn't have to separately vent the tub? I am assuming that I would have to run the tub drain and sink drain together prior to tying into the sewer line?

Like this?

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I am assuming that I would have to run the tub drain and sink drain together prior to tying into the sewer line?

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That's the way we do the bathroom drainage in my area. A single 2" vent off the lavatory through the roof or tied back to a existing roof vent in the attic. And we call that,"wet venting a bathroom group". Good luck, Tom

Awesome Tom, thanks a million!

You want to know the sad thing? The picture I posted the first time, titled "current" is how my plumbing is done now. That's how it was when I bought the house... I spent $400 on a home inspection and used a guy considered to be THE best home inspector (I'm also in the real estate business... ) and he never said a word!