Water lilyIn the winter I purchased three water Lillies

You need to get your water lilies into your pond. The plant needs to be in an aquatic baskets submerged between 6 - 10 inches in the water. The first leaves will be small and then grow larger.

They need still water to do best, the RHS has a good page on waterlilys, named varieties by size and how to grow them

As MG says, the leaves will grow. Once they have reached full size you will be able to tell how deep they'll need to be, because as a general rule, the bigger leafed ones go deeper in the water. Some, like the native white water lily (nymphaea alba) can be 10 feet down, while others, with leaves only a couple of inches across, and flowers to match, need be just a few inches below the surface.

There is a miniature water lily and the leaves are thumbnail size and don't get much bigger. Maybe you could swap it with someone who has a waterlily that is too large for their pond as this is the usual problem!

The Lillies are planted in aquatic pots with aquatic soil and slow release fertiliser, and about 3 inches beneath the water, the leaves are just so small, I think I need a trip to garden centre to buy larger specimen.

Sjw, I think you may be surprised.
The newly emerging leaves of water lilies are quite dainty. I wouldn't have given them any fertiliser, it encourages algae growth in the water which you don't want.
The water lilies will spring into growth once their roots are established and the water temperature rises. Our weather this Spring has not been warm enough yet to encourage them to put on much growth. Mine is a well established plants, several years old; I divided it early on last month and its pads have yet to reach the surface of the water.
How big is your pond? Three lilies may already be too many. My pond is big enough to sustain a school of goldfish but I wouldn't grow anything bigger than a dwarf lily in it. Within two or three seasons it swamps the pond.

I agree with Xela don't fertilise as it encourages algae. Easiest is to plant in aquatic pots in gravel as the aquatic soil escapes the pot and silts up the bottom of the pond.

Yes, the soil does escape from the pots/baskets and dividing plants that have outgrown their alloted space isn't easy either. I came across planting bags for aquatics a few years ago, they are much better at containing the soil and much easier to divide too.

Thanks for all your replies, have finally managed to add photo of pond, need to find the right Lilly as pond needs some surface cover, will be going to garden centre today. Have also used planting pockets and agree they are easier to use than baskets.