There should be 3 lap belts, one regular adjustable in the center and two retractable ones for the outside seats, the belt should come from the center and have the buckle at either edge of the seat, but yes three in total.
If you don't have them I'm sure you can pull a set from a junkyard, or since I'm guessing your kids will be sitting back there, get them new from LMC, NPD, JBG, etc.