I am trying to find out what vent on roof is the main vent to put water in to try clearing out .

There is no "main vent". Unless wet vented each fixture has its own. Tell us what's stopped up and we'll point you to the correct vent.

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put water in to try clearing out .

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That's not the best way to clear a blocked vent. Rent a snake and snake it. Put out enough snake to reach the base and 15 moor feet. Good luck, Tom

The main vent you are looking for it have to be biger then 3"

I disagree! Perhaps 50 years ago there were 4" main vents installed. N0owcode mandates that each fixture have its own vent being 2" with one vent increased to 3". As a rule we install that for the kitchen/washer vent. Tell us what's stopped up and we'll point you to the correct vent. Back to you. Tom

I may be able to use your suggestion. My kitchen/washer line may be clogged. I'll try plunging the kitchen sink while washer is backing up (from advice from a different site). When using a snake, would I need the plans for my house to know how far down to go? I've never used a snake. Do you have any other advice on using a snake?

More details please! Septic tank or city sewer? Does the sink backup when the washer drains? Does the washer back up are the standpipe? Before I can tell you what you snake from and how to do it. I need more information on what's happening. Back to you, Tom