when there is no risk of frost really. for me it is usually the endo of June but depends where in the country you are.

how many sets of true leaves do they have. that is more important really as it is the development rather than the height.

I think at 3 inches tall, you have a way to go before planting them out. You can pot them on for a little longer, each time planting them a little deeper. I would at least wait until June. If space in you greenhouse is limited, you could put them out in a week's time, just for the daytime and bring them back into the greenhouse in the evening.

Thanks very much guys, I thought you,’d say that, gotta hang on for quite a while yet.