I haven't seen that one before and will be interested to see a more definitive response than i can offer. My best guess though is that it is meant to imply you just pick the first note, at the 3rd fret, and then hammer onto 4 and then immediately after, onto 5. Kind of like you're rolling up to the 5th with 3 fingers as part of a continuous motion. This would be opposed to picking while fretting the 3rd fret, hammering on to 4th fret, then picking 4th fret and hammering on to 5th fret which might be notated with 2 lines (going from 3 to 4, and from 4 to 5).