In this format...
1st round- seems pointless as you wouldn't be able to keep them
2nd round- might consider taking a Peterson type here, but you have to project them as worth a 1st the next year
3rd round+- this is where things would start to open up for me. The biggest thing is to think about where you see them them the following year.
Since it's only 3 keepers, there isn't as much emphasis on stashing rookies like there is in a dynasty. You're going to want to keep stud veterans and may only want to keep one rookie. Obviously the later you take them, the better. In the 10+ rounds, if you strike gold, you'll lock that player down for his career and be paying pennies on the dollar for most of it.
If I were drafting in a keeper league with your settings, I wouldn't make any big reaches for rookies. Instead, I would grab a handful of ones I like in the 10+ rounds and hope a couple pan out.