SCSI Backups?

Hi, i was wondering is this is the norm or have i done something wrong? Im setting up a small backup routine (approx 150GB) to be ran every night, now i have three backup devices (external Benchmark DLT 40/80) connected to a Adaptec AIC 7892 Ultra 160 SCSI card. The software im using is Arcserve 9.0, now what im trying to do is run 3 jobs at the same time but what im finding is that i can only do one at a time. Now im figuring this is because the SCSI card can only handle one data stream at once? is this the case? if so how do i get round it? purchase another 2 SCSI cards and pop them in the same machine? Forgive me if im asking silly questions but hey we all have ot learn from a source and what better than you guys! Thanks a lot!