Is it possible all three have been in pots too long and have now run out of root room? If so, it will be quite difficult to keep them healthy looking. You can buy specialist camellia feeds (they often say rhododendron and azalea on the box too) such as Miracle Gro ericaceous, you could try that at the recommended rates.

Camellia Focus (follow the instructions to the letter) from Trehane.co.uk may seem a bit pricey with the postage added but worth it for 3 camellias.
Yellowing leaves could be a sign of mineral deficiency or overwatering. Are the pots raised off the ground on pot feet or bricks? (And water with rainwater not tap water)
What varieties are they? Not all camellias have dark green leaves and new leaves on any camellia can be quite pale, bright green rather than dark.

I think Bamboo has a point; maybe the plants have become too big for the pots and the roots with displace much of the compost. Also when a subject is in a pot and watered regularly any nutrients will be quickly washed away. If possible the plants might be better off in the ground if you have an acidic soil.

Thank you all for your advice. Unfortunately I will not be able to plant them in the ground as I live in a "heavy clay" area so they need to be in pots with ericaceous compost. The pots they are in are huge, so can't pot on, so I will try the Camellia Focus and give them the Sequestrene once a year. They certainly flower well, so can't be too starved of nutrients just yet. Thanks again for your replies.

Don't over feed - Camellia Focus provides all the nutrients. Adding other things just creates imbalance.
Mulch in spring and lift the pots off the ground (if not already done) onto pot feet or bricks to aid drainage.

Thanks Urbanite, all good advice and I have already ordered the Camellia Focus.

Good luck. Although they don't like standing in water, don't forget they need lots of water especially later in the summer when they are building next year's flower buds.