When playing against a blitz with thresh, you need to account for 2 main things. If you have the chance to push the lane and get an early level two, help your adc do so. This is huge in this match up, because if blitz pulls either of you at lv 2 you will have the upper hand because you have more cc with e and q. If you do not have the push then let it push to your tower. Blitz is almost negated if he cant pull you near his tower or middle of the lane. This also opens them up to ganks which blitz is horrible at because he has zero peel without endangering himself with an auto attack. Also, if you start hook against blitz lv 1 and you hook blitz and get him low on the first part of the game, they will usually be scared to hook someone in because they might die for it. As for playing against Janna, sustain supports are really strong against Janna. You can poke down both of them especially if the janna is selfish by shielding herself. Just keep your adc healthy and push the lane in and keep poking, they will get super frustrated. Do not try to trade with her or dive with heavy engage unless you know janna used her tornado, or you will look like a fool because a good janna will hit the nado on either the adc or the support almost every time to keep your damage and cc out of sync, losing the trade for you. Play safe and poke out of lane and wait for ganks if need be.
TL;DR: Go for early level two with thresh by pushing and immediately try to hook adc or blitz. If they are low, blitz wont pull. OR let the lane push. Blitz has no peel for ganks and is useless in protecting adc, you will also be safe under your tower even if he pulls due to your lantern. This also allows counter engage with thresh hook. Play sustain lanes such as sona nami or alistar against janna. outsustain wins the lane. wait for gank support from lane or jungle, or dont engage unless your tornado is down. I hope this helps!