This is my opinion on this and what I do.
Start with an AI game, so that you can test out the skills and try a few different builds. This isn't to really practice the champion, but so that you can use the champion without it really mattering because your gonna win anyway.
Then go onto blind pick, and just instalock it in. If you are forced to use it in a lane that isn't really where you should be its fine, because this is more about practice, learning the tricks of the champion, and using it against others. Then once you feel like you have a good grasp on things like last hitting and stuff, you are now ready to add it into your box of useable champion for draft mode.
However. If you only know 3 champions you wont' be ready for ranked at 30. You need to have at least 4-5 full (different) rune pages and should know at least 3-4 champions for each role. Ranked is done with draft mode rules, meaning picks and bans. Knowing only three you could get banned out, or have those champions picked already. (Morgana is sometimes an isntant ban, and ashe and mundo are pretty popular.) The hardest part about this to me, is the rune pages, they are expensive to get the pages themselves, and can be twice as much if not more to fill, so we're talking like around 100K IP for 5 pages of runes that are different. It's really lame, but there are reasons for it.