What is this technique how do you do it?

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It looks like a Hammer on 4 and 5 with a line connecting 3 to 5 wanting you to do something. I can't make any sense out of it.

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).

Yep, it means you pick the 3rd, hammer 4th, then hammer 5th.

Being TAB it gives no indication of timing, though. The delay between each note could be anything.

One of the reasons I like this site is because both TAB and Manuscript notation is given; even if you can't read the manuscript, you can get a feel for the timing on the TAB if you understand the basics.

DraconusJLM has the right answer. Pick 3rd fret, hammer 4th fret, hammer 5th fret.

It's legato playing. The line connecting the 3 notes means they are connected / slurred. Rather than picked individually.

-Carl.