They surely couldn't expand T3 whilst the TP work on T2 is being undertaken.
Current proposals indicate that the TP will be looking at completion around 2023 with T3 redevelopment coming at the back end of that timeframe based on west-to-east construction. I don't believe that leaving T3's capacity crunch unaddressed for that long is a serious option for MAG.
No doubt you are aware that during the run-ins to both Summer 2014 and Summer 2015 there were strong rumours that Ryanair would base a larger number of aircraft at MAN than eventually proved to be the case. My impression is that the desire to expand is genuine from Ryanair's point of view, but they will only do so if their flights can be accommodated on T3 terminal stands. Remote parking and bussing simply doesn't cut it for them. In Summer 2015, there was great confidence expressed from some quarters that Ryanair would base 9 or 10 aircraft at MAN ... the eventual number was 8 (same as August 2014). And the T3 stands were fully subscribed each night.
Ryanair has recently stated that they anticipate 33% growth at MAN 'next year' (did they mean 2016 or 2017 by this?)*, and as you know, another site has again discussed an increase to 10 x based B738's. Aircraft visiting from other RYR bases can only do so much. FlyBe (incl Loganair) is growing too, whilst Oneworld (BAW, AAL, FIN, SUS, IBS) and Air France/KLM also operate from T3. Also Vueling and Adria.
Given the above, can MAG afford to leave T3 essentially unchanged until beyond 2020? Not if they want to harness the growth offered by Ryanair and FlyBe. Something has to give, and soon.
What are the options for a quick fix? Relocating Oneworld and Air France/KLM to T2 would be ideal, but those carriers appear very happy to remain in T3. Can they be persuaded to move? In the longer term (with US PDC established in T2) probably yes. But something needs to happen well before then.
Another option is early removal of the airside demarcation between T1 and T3 allowing much greater flexibility of terminal stand allocation. This will happen eventually anyway, as the plan is for the entire terminals complex to be linked throughout airside. Early linkage of T1 and T3 could be MAG's best bet at this stage.
Obviously, the third option would be to construct / enable additional terminal stands at T3, dusting off old plans which allow for this. But - as you say - MAG would be understandably reluctant to do this in parallel with the TP over at T2.
But, if MAG wishes to retain the tailwind offered by MAN's most rapidly-growing based carrier (Ryanair), doing nothing till 2020 or later is simply not an option. Ryanair and FlyBe growth will stagnate at MAN if T3's capacity limitations physically curtail their ability to expand. The crunch already looms. A solution is required within months, not years. It will be interesting to observe how MAN intends to address the challenge ahead.
*EDIT: Having read another source, I now believe the RYR growth projection relates to the 2016/7
financial year.