I think to make it work, you need to keep busy during their day.
If you stay home alone and moping, you'll miss him more, be more jealous, and it won't help any.
If, however, you see it as a free day for hobbies you like, for going out with friends, for things you really, really enjoy doing, and you fill your day with them, not only will the time go by faster (and with less jealous thoughts) but you'll have a bunch of things to tell him the next day, such as the movie you saw, or what you did during your hobby, or what a friend of yours said to you...
It can be hard to think of things to do when we're focused on one thing we wish we'd be doing but aren't. So whenever you can, try and write a list of suggestions to yourself. Then when you feel restless, lonely and jealous, take a look at the list and do your best to do some of the activities and lose yourself in them. It will help.
As for the texting, you should probably talk when the three of you are together to see what amount of contact is appropriate. When he's with his other boyfriend, if that boyfriend is upset by texting, it won't matter that you aren't. See what he'd be fine with and stick to that, and expect the same courtesy from him. But first you have to know what is or isn't okay, by talking about it.