Personally, I wouldn't put three plants in a grow bag and 5 litres is very small.

I use 12-15 litre pots for a single (small or standard fruit) tomato plant. It can still be difficult to keep them moist in the summer.

Try the capillary matting approach, but be warned that Watering from underneath is much less successful with larger pots.

Do you know why didn't your drip watering work? They do need some care and attention in setting them up.