The main differences are the filter volumes, flow rates and therefore the tanks they are rated for (to convert to US gallons divide by 3.78
2222: filter volume 2.3l, pump flow rate 500l/hr (350l/hr effective rate), rated for tanks up to 150l.
2224: filter volume 3l, pump flow rate 700l/hr (500l/hr effective rate), rated for tanks up to 250l.
2026: filter volume 4.9l, pump flow rate 950l/hr (650l/hr effective rate), rated for tanks up to 350l.
Both have filter media baskets, the 2222 and 2224 use 12/16mm hoses while the 2026 uses 16/22mm.
The other difference is that the 2026 has the built-in priming pump but I never use that feature anyway as I don't find it works very well.
They are all great filters.