I dunno, formidable could definitely apply
"inspiring fear or respect through being impressively large, powerful, intense, or capable."
The three of them would definitely be very intense and whilst I don't see them attracting much respect from SNP I wouldn't fancy them on the opposition benches. Salmond's a knob but he's a good orrator and a very seasoned parliamentarian who knows how to debate and would be quite capable of making verbal mincemeat out of many SNP politicians. Can you picture, for example, Salmond vs Humza on the HCB?