Very late to this thread but yes, it would be unreasonable. I say this as a mother to a baby.
The reality is, I once had to queue for a mother and baby cubicle. My DD at the time was only a few months old, and there was absolutely no other workable solution (the car was parked miles away, only toilet nearby, etc). No word of a lie, but we were queuing for over 15 minutes for one person who had two kids. The fact you have 3 children and are wanting to use a disabled toilet is ridiculous.
I didn't enjoy waiting, but I had to. However, I cannot imagine how awful it would be if someone who urgently needed to use the disabled toilet couldn't because you were in there with three kids. Can you not see how the logistics of this would work? You wouldn't ever just pop in to change one child quickly/have a quick pee yourself. You'd go in, change all three and go to to the loo yourself to save yourself hassle later on. Therefore, leaving the toilet out of use for at least 15 minutes.
If you had a disabled child or were disabled yourself, it would be fine. However, you aren't. Leave disabled facilities for those who need them. You chose to have three kids under 3, no one made you do that. You have to come up with your own solutions that don't involve impacting on someone else's life, especially when you have no need or use for the space you're occupying.
Honestly. Next you'll be wanting to use a disabled parking space because all the mother and baby ones have been taken and you have "three under three". It isn't an excuse.