So the dad of your kids will be in the picture for max.15 years...
If first born is born in the 2nd yr of ur relationship, feel free to introduce the toi to its dad thus: 'hey this is going to be your daddy for the next 13yrs!'
Kid No. 2 (assume comes in say 2yrs after 1st born i.e ) - tell toi no. 2, hey this is going to be your dad for the next 11 yrs!
Kid No. 3 (assume comes in say 2yrs after 2nd born i.e ) - tell toi no. 3, hey this is going to be your dad for the next 9yrs!
Is this what you want? Is this what the kids would want?
As an adult, it is okay if you want to live this life, but don't drag the kids in.