I only know how to play veigar, and this is how I usually fight a Leblanc...I kinda know Ahri
Veigar, play it safe, killing minions with Q to build up your passive. Once you hit level 3, if they are stunned do w + q, repeat this a couple times until you have a decent amount in your passive. (Like 20-30) Never attack the enemy turret unless you have ghost/flash ready and your E isn't on cooldown.
Leblanc has high cooldowns and not very good waveclear. Be careful after you do a combo, enemy might try to attack you knowing you can't fight back. Also, watch out for stuns. If you W someone and they stun you it will send you right back to your original location.
With ahri you just poke and poke and poke. If you don't land charm, then chances are they're going to be trying to attack you. Try to land your Q at it's max distance so the true damage and magic damage hits at the same time and they can't dodge the sphere coming back. Use W if they come close. Only tower dive when you reach level 6. If you use all your charges on your ultimate early, stay near your tower or your half of the lane.