Anytime an engine mount has a crank in it, it should be taken care as soon as possible. It will only get worse. Since it is a front mount, that means the engine is probably starting to move more than it should when your Honda
is put into drive, or when accelerating. The front of the engine will be moving upward. That is the direction the engine wants to move. In reverse the engine wants to move the other direction, so no problem there.
Most likely the reason the dealer mechanic
wants to replace the others is because when one is bad, it tends to put more stress on the others. So he may have saw the others were weak, or he just suggests it as preventative.
If you do not hear any loud banging or feel jerking when putting your van in drive or when hitting the gas, i would just go ahead and replace the one that is broke and keep an ear out for any nosies in the future.