Have you tried sowing several seeds in each module? I don't mean sowing seeds of the same variety but seeds of different varieties that would compliment each other in their growing spot. For example you might sow a lettuce, spring onion and parsley in the same module or beetroot, chives and rocket. If they all germinate, they're planted out together. If only two germinate, its still good.
If they're seeds that crop at different times its a bonus.
What do you think? Plan or No Plan?
That has potential. I like that idea. I don’t see why it wouldn’t work.
OK, so your 3 seeds all germinate in a 2" sq module. I'm not sure whether a parsley and lettuce would cope being in such close proximity over time. I saw somewhere sowing 2-3 onion seed in the same module and planting out without separating could work but giving smaller bulbs. Would your resultant baby plants in the module be eventually stunted for lack of room? You know parsley and some lettuce are rather blousey and like to flounce. That reminds me I might get some onions sown today if it stays fine.
Parsley and lettuce were just off the top of my head suggestions - it could be 3 anythings as long as its not 3 roots. They need to either take up different growing spaces or be picked before the others need the space - so you might pick the parsley while the lettuce is growing.
I think it could work if you used something like a mesclun mix, then it'd be sort of like those living salad mix trays. For bigger plants which you don't harvest regularly to keep small, I think the faster growing plant will swamp the slower growing ones. Onions are very slow to get going and would disappear next to a lettuce.
I do interplant spring onions between lettuce and other salad leaves, but only in the middle point of a square foot and when the onions are tall enough to compete for light.