Will Apa 3 help a European Finch that has been ill for 2 weeks already with half-water, half-solid faeces?

The fact that part of the faeces are like water makes me think trichomoniasis, so yes, APA 3 is a good idea. (Otherwise if you already have Plasmatox and Nuovo Tricofood then you can use those together.) Provide at the dose of 2mL per liter of drinking water for at least 2 weeks (until the finch is back to being well).
If the finch shows a dark line in the liver area, then there may be a simultaneous liver intoxication, so a 30-45 day cycle of Apasyl would be a good idea, too. (This problem is very common in finches).”