How much am I allowed to gift my other two children, after the third one bought my house? I want to spend down for Medi-Cal.

Sounds like you need to get a lawyer who specializes in Medicaid/estate stuff involved.

There is no problem selling the house if she got Market value. She will be penalized for the difference between Market value and what she received. Its giving of "gifts" that would be a problem. If done before the five year look back then really no problem.

Keeping a house is not good either, in my opinion. Once on Medicaid, none of her money can be used on upkeep or utilities. Unless a family member wants to pay the bills but no guarantee that when the house sells the person will be reimbursed.

Oh dear :(

Can s/he buy the house back from the child? Would that be sensible, if it's possible?

Nothing.

Selling the house is probably the worst thing to do if you are looking to be on medicaid. Your primary residence was exempt from consideration for medicaid. The money from the house sale isn't. Since you are in California, you could have left the house to your children where it would have been exempt from medicaid recovery. Now you have a pile of money that you will have to spend down to qualify for medicaid.

You can’t gift them money during the spend-down. The money has to be used toward your care or you will have to wait even longer to become eligible for Medi-cal.