ok first i want to tell you not to get discouraged as that technique (playing with 3 fingers) take a long time ti master. galloping is the first thing you'll be able to do but what's really harder is playin notes constantly in groups of four as opposed to groups of 3 like galloping. I strongly suggest you to get that technique down otherwise when you'll want to play really fast fingerstyle it'll sound much more cleaner with 3 fingers than 2.
first you need to know which patern is natural to you. you can do index-mid-ring(123) or ring-mid-index(321) i'll refer to that as 123 or 321. just grab your bass and try it there's one direction that'll fell more natural that the other. for most people it is 321 but for me it alway was 123 from the start so go figure I guess it really depends on the person.
Now you need to remember that before a finger hits the string again all other fingers need to have done there patern example if you use 123 and play notes constantly without stopping you should do 1231 2312 3123 1231, I made groups of four because in rock music 4/4 is the most used time signature and there's usualy eight note by bars (if your playing eight's wich is current in rock music)
the thing you dont wanna do is alterning with your middle finger in that pattern 1232 1232 1232 1232 it might seem more natural but there's no gain in speed since your always alterning you middke finger with an other finger that way you can't play faster that with 2 fingers the only benefit is that your index and ring will get tired in twice more time.
i'll give you an example of an iron maiden song (it's what you wanna play right

)
it's the trooper, the main riff (bare with me i'm goin to tab a part of it but i do it by memory since and I don't have my bass with me, it could be wrong.
g------------------------------------------------------
d------5--------------------------------------5--------
a---5-----5--7---777--777-777 777 -777-77-5---5-3---
e------------------------------------------------------
____i__m__i__m__imr___imr_imr__imr__imr_im_i_m_i_m
note: for the slow part i use 2 fingers because it sounds more consistent.