the simplest solution is to bypass the combiner : you gain 3dB in downlink.
but then, your uplink will be limited. Therefore, a LNA (or TMA, that’s the same) should be implemented. I don’t know about LNA faults, as Mania suggested. Get a well-known brand I guess.
But in the en, it’s a lot of trouble to gain only 3dB !
All considered, and as Mania said, use the Twin TRX in coverage mode (that’s an Alcatel solution, but i think other vendors also propose something like this, don’t they ? one twin TRX connected to 2 cross-polar antennas : 2 Tx-div + 4 Rx-div, and a huge Tx power…) Here, no need to use HGA : the power is so high, with diversity on top of that… that’s plenty enough. Expect 10dB increase !
Another solution would be to use an indoor repeater, with a donor antenna outside. That might be too expensive though ?