How do 3 phase meters work with single phase solar?

If you have single phase solar on a 3-phase supply then there are 2 different realities happening regarding imports and exports:

Reality #1 - the electrical reality.

If you have a 3-phase load pulling equally on all 3 phases - let's say 2 kW per phase, and all your solar inverter(s) are on a single phase and you are generating 6 kW on that phase. In actual fact your solar is working directly to meet the 2kW load, the remaining 4 kW is being exported. The retail meter can only see that one phase is exporting 4kW and 2 phases are importing 2 kW each.

Reality #2 - the billing reality.

Inside your retail meter all 3 phases are 'netted out'. So while theoretically you're using 6kW and also exporting 6kW, for billing purposes all your retailer knows is that two, plus two, minus four... is zero.

So, the net result is, you are not exporting or importing. The meter "balances" the raw measurements of energy flow before applying the tariff charges. And the good news is your bill will reflect that.

Reality #3 - on balance

If possible, it's better to have solar on all 3 phases of your connection, be it a 3 phase inverter or individual single units on each phase. Meeting your loads head on, having consumption addressed with generation, means the retail meter doesn't get involved, so measures of power factor or throttling because of uneven voltage simply don't arise. You'll get full benefit from your own energy.