Multi-tabling can benefit you because you'll be playing more hands/hour, which can mean more profits. However, in your case, I would suggest not multi-tabling because you dont have the BR to handle the swings that will come with multi-tabling. Also, you said that you are trying to follow the 5% rule, but 3 tabling with a $50 BR isnt exactly doing that. Its 5% of your total BR, not the buy in per table.
You dont have the BR to play at the limits you are playing but, as you said, those are the lowest. Buying for $2.5 at a .10/.20 only gives you 12.5BBs to play with, which isnt a lot to play with and doesnt give you a lot of mobility. You practically have to get really lucky and thats what I believed happened in your big run. ($2.5 - $15) However, runs like these arent going to happen all the time and you will experience downswings. Which is why you need to aim for a more stable bankroll and not focusing on increasing it so quickly. If you're playing solid, smart poker the profits will come; sometimes it'll be slow and other times you'll have good runs.
So, my suggestion to you to not worry about multi-tabling right now. Just focus on building your BR with proper BRM. Once you have a solid, stable BR you can try multi-tabling because you will be able to handle the swings and such.
Have you tried playing SnGs or MTTs to build up your BR. A good cash in an MTT could give that BR boost you're looking for.