One of the keys to progressing from 2*s to 3*s is to not rely on your 3* characters too early. A champion 2* (level 94 and higher) will be more powerful than a 3* with few covers. Generally speaking, a 3* character starts becoming really useful once they have 10 covers, and can then be leveled up to 127. There are exceptions, such as characters that do not do major damage and instead focus on board manipulation (in which case leveling them only gives more health points), but for the most part you are leveling up characters for them to both do more damage and have more health points.
Some players only level up 3*s once they are ready to be championed, but leveling at 10 covers gives you a 3* character that’s more usually powerful than your champion 2*s, and allows you to spread out your Iso to a larger number of characters. 72% of the Iso you invest into a 3* champion is from level 128 to 166. So bringing up as many characters as you can to level 127 is the most efficient way to get usable characters.
As for who to level when, you have two resources. One is the 3* Character Ranking here on MPQtips.com. But something you will also want to watch closely are the weekly buffed characters. Each week, five 3* characters are buffed, meaning they are boosted to higher levels than their regular levels. If you focus your leveling efforts on the characters that will be buffed soon, you will get some extra powerful characters for that week, and get the most out of your Iso investment for the short term. To see who is up for being boosted soon, check out our Weekly Buffed Characters post.