Why does my front shifter take so much effort to get into 3rd?
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.