This seems like an awful journey, you won't sleep at all and you'll be exhausted the next day (at least...).
The first 2 trains are night trains, reservations are required. However they might not sell them or let you board at 3am.
EDIT : they do not.
The third train shouldn't be too busy. You can make a reservation on the Austrian website (ÖBB) for much less money: 3€ per person incl. seat selection with a map.
Honestly please research another itinerary. There are 2 very good night trains between both cities, leaving at around 6pm.
General guide for the best places to book: https://www.seat61.com/interrail-and-eurail-reservations.htm
Here are 2 good options (book ASAP):
- EN Praha hl.n. - Zürich HB via Austria 18:20 - 08:20 200€ for a sleeper with ensuite WC/shower (100€ pp)
- EN Praha hl.n. - Zürich HB via Germany 18:28 - 09:05 196€ for a regular double sleeper (98€ pp)
Both with comfortable private compartments, I've just tried one last week! Definitely worth the price for a good experience.
For the latter train, couchettes in shared compartments available for 54€ pp.
Feel free to ask anything.