What do you think 3% LS should be worth in terms of dps?
I think that this is a hard question to answer.
The lower your eHP (as long as it's not too low) the more the LS is valuable. I remember a long time ago I could kill myself in under a second without LS against an RD mob. But these days I have a couple seconds even without LS.
It's sounds kind of weird but IMO the lower your DPS the more valuable LS is because it's pretty cheap to replace a couple thousand DPS in another slot, and as a percentage, the raw number is much lower.
I remember when I was at 100k dps (raw), I felt pretty comfortable giving up 5k dps (ie 5%) for lifesteal.
Now sitting on 230k dps, the thought of giving up 11.5k dps (approx 5%) for lifesteal is a little harder to swallow, though I think I'd still do it. Before I tried it out, I thought I'd probably
prefer the 22k dps option over the 2.8% LS option (at 210k dps), but in practice I didn't realize how not-close it was. The 22k wins hands down.
I'm not sure how I would model LS from a blank slate, and I'm not sure that having it be a set constant percentage of DPS is the best model, though I've thought about it and can't offer a better model.
I think that 10% of DPS is still high for the most part but something I can accept as within the realm of reality. As Quos noted indirectly, LS and LoH are better if you are using a bow, and even more better if you are using 1h xbows. And LoH scales inversely with DPS (or more specifically scales poorly as damage per attack goes up, it's just that often attack speed stays relatively constant (barring type of weapon switching) while damage will increase substantially as your gear improves.).