I seem to recall this happened last time SL toured Aus too, we started with Kulesakara, Welegedera and Eranga and ended up with Pradeep, Lakmal and Prasad, but at least had phatty wheeling away. If you go back to the NZ tests Rajitha was actually the incumbent 3rd seamer so if he and Lakmal play that's still fairly close to a first choice opening combination, but damn, I reckon best bet is play the all-rounder they've flown in at 8 to bolster the batting, rely on DdS for some offspin and just hope the batting turns up and the bowlers manage to find something under lights to keep the contest somewhat engaging. If the pitch looks like it might turn a bit I reckon Sandakan should come in, Dilruwan Perera is not batting at anywhere near his 2017 peak (where he was almost a legit no. 7) and contributes very little with the ball, even against England at home he made little impact against the left-handed heavy line-up they brought. He's also like 35 so I see no future, he should be gone by the time SL does another world tour