3 PR6 links means your title going on three pr 6 links. means more serps..... means more chance better in your Keywords.
Personally I would definately go with the 3 PR 6 links, presuming they are on different c class ip blocks. You will get a much better value from your paid for links.
For PR you should also consider the number of outbound links of the sites in question. And in general you should also consider visitor numbers and link position.
If you talking from only PR value here is math toolbar PR is calculated based on log of page rank number base to 5.5 now average pagerank number for PR 6 link with 50 links is 1507.48 so will will get about 3 * 1,507.48 = 4522 (toolbar PR 4 range) while if PR 7 with 50 links is poimting to you it will pass 8,291 (toolbar PR 5 range) so from PR point of view single PR 7 link is better (i am assumming average of 50 outbound links on page and mean value of PR ranges) But from SERP point its better you go for 3 PR 6 links
Hmm... Seems like you're absolutely confident about this... Do you have an insider at Google? We have all read Wakfer (as well as Bob Mutch ) who uses 5.5 in his calculations, but, for example, Sobek says that the logarithmical basis for this scheme should be between 6 and 7... And reality shows that the algo is much more sophisticated...
I'd definitely go with 1 PR7 link instead of 3 PR6 If you get 3 PR6 BLs in a short period...you'll be suspected of buying links maybe
Yes, Google tries to devalue bought links (as well as any other unnatural links), however it's unlikely that this happens with just three links unless they're labeled "bought links" or something.