Overlapping implants is one of the reasons we see bullers.
If you want your implant to last 90 days, use Synovex S or H instead of Ralgro.  The only place a Ralgro will last 90 days is on grass.
You can use Synovex Choice on both steers and heifers, so I'd recommend that instead of the Rev IS.
If you buy weekly, I'd wait until I have a months worth of calves and then implant them with the Synovex S or H.  Than I'd mark on the calendar and implant that group every 90 days after that.