Hello - I have 18 year old triplets...

You lost the child tax credit for all of them in the tax year in which they turned 17.   From now until you cannot claim them as dependents, you get the $500 credit for other dependents.   There is no special federal tax credit for saying that one has a disability, unless you have to pay someone to take care of the disabled child so that you can work--and then you can use the child and dependent care credit--but that credit is not something to enter on a W-2.

If they are full-time college students you can claim them until they turn 24; if they are not students they can be claimed until they are 19.   Depending on the severity of the disability, one of them might still be a qualifying dependent after the usual age limit and you would continue to get the $500 credit.

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2024 taxes as long as they meet the following requirements:

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
• No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year (exceptions apply).
• They made less than $5050 in 2024 (not counting Social Security)
• You provided more than half of their financial support.