Integrating user acounts
Each time do you a migration, it will make a new account on the new machine.

I can only assume to get 3 separate accounts you started with a brand new user on the new Mac, then did two separate, possibly partial, migrations.

It is possible to manually merge them, [I also learned the hard way how to do this] but in all honesty, &amp; especially as you claim to not be a power-user, it will be simpler to wipe the Mac &amp; start over. It may not actually take any longer than trying to merge what you have currently &amp; is likely to be less 'hazardous'.
Do a full single migration as part of the initial setup. When the machine first asks you who you are, there is a point at which you migrate instead of making a new user. 

Apple have a step by step procedure to "sell" your Mac &amp; set up fresh for the "new owner". I would follow that. What to do before you sell, give away, or trade in your Mac As it reboots once more as a "new Mac" then follow the Migration guide - How to move your content to a new Mac which should then Migrate to one single new account, matching the one you had on your previous machine.