Well, I'll have to disagree with your teacher. Do aim at one finger per fret as a default!
The three-fret technique is essentially a variation of the standard way to finger notes on double bass, due to Franz Simandl. Double bass requires about five times as much force as electric bass (apart from having a considerably longer scale), so it's really tough to apply the 1-finger-per-“fret” principle. Some guys can actually pull this off and achieve marvellous results, but for most players it's just not practical to torture the ring finger and pinky with having to work on their own; the Simandl technique groups them both together to a single, more powerful unit.
This is completely different on electric bass. If you have difficulties pressing down the string with the ring finger or pinky alone, then there's either something wrong with you technique or the action of your bass is set too high. Indeed, it's very rare for players to actually use the pinky and ring finger together, rather they'll just skip the ring finger.
So, force isn't really the issue.
What's quite a different issue is the spread of the hand. It's a matter of fact that even on electric bass, the position where your index is on the first fret while your pinky fingers the fourth is extremely uncomfortable. Don't do that, ever! You don't need to. No matter where your hand is and which finger targets what fret, the hand should always be nicely loose and flexible. Don't spread out fingers while they aren't actually used; only keep them ready. While pressing the pinky, the whole hand may in fact be a good bit higher up the neck than when the index finger is playing. It's still nominally the same position, and the move of the entire hand within an inch won't slow you down.
It turns out many bassists, by being lazy and using three-finger technique, actually miss the goal of a relaxed hand. The three-finger technique, like the chord technique used by many guitarists, has a tendency to encourage a firm broomstick-grasp like grip. Actually this kind of technique can give good results (especially à la Hendrix – screaming bends and thumb auxiliary bass notes), but on bass it's most of all limiting. A more open hand is much better for melodic bass lines, not always just playing pentatonic scales or merely switching between fifths, and it gives a better basis for switching to other positions.