I have 8 year old female 12 year old male and a 15 year old female there mother has them all in one bedroom. Is there any kind of law against this?

No, while it is not the best, it often is called being poor and not being able to afford a larger home. We all often just do the best we can.
If they have proper ability to change clothes in private and are not in the same "bed" ( twin beds) but in years past several people, had to share rooms, often children in room with parents.

This is not work if the family were being looked at to adopt a child, but often it is how clean and how well adjusted the kids are.

Do you rent or own? What is your country of origin?

Do you rent or own? What is your country of origin?
I own and live in Stanislaus county California

No, while it is not the best, it often is called being poor and not being able to afford a larger home. We all often just do the best we can.
If they have proper ability to change clothes in private and are not in the same "bed" ( twin beds) but in years past several people, had to share rooms, often children in room with parents.

This is not work if the family were being looked at to adopt a child, but often it is how clean and how well adjusted the kids are.

Is the fact that I have a four bedroom house with rooms for all going to make any impact? These children were removed from my custody over a lie. That lie has been proven to be a lie. Is the fact that I can better provide for the children going to matter?

I think more info is needed as its confusing to conceive of crowding kids into one room if there is room for spacing them out. Also according to the HUD standard, two people to a room is acceptable for landlords.

Were your children being removed from your home have anything to do with them all sleeping in the same bedroom? While you have the space for each one to be in their own room?

Were your children being removed from your home have anything to do with them all sleeping in the same bedroom? While you have the space for each one to be in their own room?

No it was false allegations about domestic violence. Charges have been dropped and there is more evidence to go along with that to support that it was a lie. As the ex wife was involved also with these false statements.

I doubt them being in one room will effect a custody hearing, it may be used by the other person to ask for more child support, or any child support you are behind, I can see them crying to the judge, if they only paid more we could afford a better place.

But life style, child being at risk, in danger all have more to deal with custody. Also at 12 and 15 the children should be allowed to testify in court, so if lies were being told, did the children agree to the lies ?

You will need a lot more against them than this, it will add a little if you show other neglect, and if they will testify as to where they would prefer to live.

But you will have to address the issues of abuse that have been raised, since they removed and changed custody of the children, the courts believe this is true, so this will have to also be address to get them back. This is way beyond any help except from a local attorney that you will need.