Budget £10,000 for an annual (in case you get a bad one), £800 for each 50/6 month check (at least two of those), £1,600 insurance, plus whatever parking you can negotiate and you start to get an indicative cost. Say £100pcm to park it, if you get a good deal, another £1,200 - that lot above is £4,400 each per year, just to keep it standing still.
You then need to decide on an hourly rate. That will cover fuel, say £50/hour, plus you'll need to allow some money from your hourly charge to build an engine fund, pay for unscheduled maintenance, upgrades/options and a few unforeseen bits and bobs too. Even so, your hourly rate will still be much less than a rental hourly rate. It should come out less than £100/hour.
How many hours a year do you think you'll fly? Divide the £4,400 by that and decide whether that figure (per hour) in addition to your hourly rate is worth it to you to avoid the club rental scene.
I personally don't think three person groups are economical if you're flying less than around 100 hours a year. But, you can't put a price on the freedom of ownership and escape from the club rental scheme, it's a hobby after all and you sink money into hobbies to have fun.