It's possible to get three days of positive OPKs (or in your case, get them over a span of three days). My guess is, if you got a positive on Tuesday, a negative on Wednesday, and a positive today, that your urine could possibly been too diluted to test positive on Wednesday. OPKs show the LH surge which is typically 12-36 hours before ovulation. The fertility window is generally the three days before O, O, and the day after O. Without temping you can't know exactly where you are in that process, but having sex everyother day is fine. I would for several days after your last +OPK.
Best of luck!