Gooseberry questions

sawfly - use an ordinary bug spray - sawfly comes 2 or 3 times a year so watch out!
birds - it's the birds that steal the berries - cover the bushes with netting to keep the birds out
mildew - you can cut out any bits affected by mildew - gooseberry bushes are pretty tough - mine have had mildew and are still producing
spacing - 3 together to make one large bush is probably fine, not likely to do any harm - but as with everything else, you could improve yield per plant by careful spacing and pruning etc
pruning - from the bbc gardening website:
i have 3 bushes at home, all been attacked by mildew and sawfly every year - i haven't pruned much although have taken out some of the worst mildew and the centres are wide open - they are very scraggy and untidy but still produce fruit which is normally snapped up by the birds before i get any

i have 2 large bushes on my allotment, both about 3ft wide by 3ft high, fairly rounded shape - the centres are not open but they look good and producing a lot of fruit

I have gooseberries about 4 foot apart and they still get entangled. Maybe yours was originally a single plant that layered itself? ( they do this quite easily where a branch touches the soil)

Once I've taken cuttings I would like to try growing some as standards on a single leg which should keep the fruit and branches away from the ground!

Likewise with whitecurrants!