I have 3 children he is receiving diability around 12000 a yr. I work and have the custody of the kids and make before taxes 30,000. He says he can get alimony because we were married 18 yrs.

Alimony is support paid to a spouse upon a divorce. If he made significantly less than you and relied on your income for support, then he might be able to get alimony. This is very dependent on local courts and judges as the courts have some discretion in ordering alimony. So you need to talk to your divorce attorney about this.

As noted, it will depend upon the law of whichever state or country OP is in. And one shouldn't confuse alimony with child support. They are entirely different. Alimony is to the ex spouse, while CS goes to the custodial parent for the benefit of the children.

Alimony has nothing to do child custody and child support. Yes, they may be able to.