Quick point of note. If you read the PSO award notice, you will see why bmi regional are operating only twice daily, not three.
Both bmi and another bidder, by capacity would suspect flyBe, bid over the maximum subvention limit, thus a compromise was agreed with the lower bidder, bmi, in that if rotation dropped from three to two, they would fall within the subvention limit and therefore be granted the PSO.
I suspect bmi will not be in any hurry to therefore add a "3rd" rotation unless either additional funding was offered or they really felt a compelling need to add, such as towards the renewal period to put off other bidders with 50-seat units, or commercially the rotation would more than cover itself.
The caution would arise on the basis that if the PSO funding paying for the unit and crews was then seen to subsidise an extra commercial rotation, or indeed artificially support an alternate route, which is where such funding mechanisms prohibit growth rather than encourage under EU state funding regulations.