I have been asked to take out my friends 2 dogs and a relations dog (all dosile lovable dogs) when I take mine out, they want to pay me a few pounds for taking them out, the thing is... if they cause an accident or attack another dog am I lible or are the owners, I need to clear this up before I take them anywhere. I would also need the name of a company that would cover such an insurance

As the one walking the dogs I would get a waver signed. Stating that should their dog attack the another dog they a responsible, and should an accident accrue they are responsible for 1/2 the expenses and you the other half, and should another dog attack their dog the owner of the other dog is responsible not you. I have walked dogs for neighbors and all of them signed a waver. I never needed it but at least I was prepared. the only insurance you can get is business insurance ans then you would need a license, but for only a few dogs I wouldn't worry about it.

this is an iffy situation because they are friends or family members they may not care, (until something happens of course) You can do one of two things, have them sign a contract saying that you are not financially reliable if something happens to the dogs while they are in your care.
or get insurance.

If any money is changing hand(even if its cash in hand)you must get insurance,google public liability insurance then ring round,if you are just going to stick with walking no more that 2 dogs(including your own)you might be ok so long as the other owner clearly knows you take no resposibility.

contact your lacal trainer for advice on the best insurance as yes you should have some
you need cover for accidents and your friends should have pet insurance any way
think of it more as a safety net rather than an admission something could go wrong