Could someone tell me why this question received 3 downvotes yesterday? I really thought it was well asked, it surprises me. Moreover, it received 3 upvotes before, so I believed it was alright.
No one except the downvoters themselves can tell you why they downvoted your question, and if they didn't leave a comment, it's not particularly probably they will answer here instead. However, there are currently two critical upvoted comments on your question (whether they are from downvoters or not) that you haven't addressed in any way.
In general, if I get a comment that says something like "I don't understand this" or "This doesn't make sense", then I'm not surprised if I receive a downvote afterward, and I'm not really sure why you are.
You started a bounty on this question. This exposes the question to more views and hence also more votes - you got +2 votes before the bounty, and +2/-4 votes after the bounty at the time I'm writing this answer. So people's opinion about your question is split, and I wouldn't see it as strange that you got a few upvotes before the first downvote arrived - I mean, some of the votes have to come before the others, and statistically there's no reason the downvotes should arrive before the upvotes or vice versa.
Furthermore, people tend to look at bountied questions more critically: Not only do these questions grant a lot of rep to people answering them, they are also much more visible. I suspect that people might be more willing to downvote a bountied question that they find pointless or uninterested than they are to downvote a normal question that's just not very interesting to them, but I can't really prove this assertion either way (and I personally try not to let the bounty status influence my voting).