Why does my front shifter take so much effort to get into 3rd? I'm trying to get my son's bike dialed in.  We bought it used and it's not quite right with respect to the front shifting.  I got the front derailleur stops adjusted properly so it limits at the right place when moved by hand.  I fought with the cable tension until it was able to shift into 1st and 3rd, but even so, it takes an incredible amount of force to push it into 3rd. Specifically, I push with my thumb and it clicks into 3rd at the shifter, but I need to push a little past to get it to actually shift. Pushing out of 2nd into 3rd, it resists strongly such that my thumb actually hurts from debugging this.  I don't think my 12 yo will be able to do it.  What might I have done wrong?
It's Shimano STX, thumb/finger shifter.
It is difficult to say without physically seeing the bike and the integrity of the cable to the derailleur. One thing, I can think of is that there could be too much slack in the cable, which is why there is the need for an extra push. However, with a triple chain ring if you have a good shift from inner to middle that perhaps is not the case.
Also to go from the inner to outer chain rings you are putting tension on the cable, which is why you feel the effort more as you go from inner to outer chain rings.
I will say on my road bike and a long time ago on a kids mountain bike with triple chain rings I do recall when going to the largest chain ring it was not like a click + shift, but a longer push and shift similar to what you are describing (i.e. I shifted and had to almost push the shifter more to put enough tension on the derailleur to move into the new chain ring).
Most local bike shops will be able to do a quick safety check on the bike and perhaps can tell you if it is normal. That is probably where I would start.
Hope that helps.