We've got a 4kW PV array. On a good sunny day (east coast of Scotland) it will generate 25+ kWh. Over the course of a year it will generate between 2500-3500 kWh, or units of electricity, the UK average per kW is apparently around 700-900 kWh per year.
Electricity at the moment is about 14p per kWh to buy.
So your 3kW PV system, assuming a conservative annual output of 800kWh per year per kW, will generate 2,400 kWh per year, or £336 worth of electricity. Offset the feed in tariff (16p now, I think?) and account for the capital cost, and you can judge whether it's worth it.
You can be almost certain the cost of electricity is going to rise over the years, and the FIT is index linked, so the margins should get better as time passes.
Can't comment on solar water. We've got a ground source heat pump and it couples really nicely with the PV.
HTH.