BIG WARNING BELLS
I always shudder when I hear people wanting to do what you state
First of all, the bottom boom of the trusses ( that's the bottom horizontal member ) in your home are probably not designed to carry the load of a floor. Given the shoestrings that most contractor's build by these days, They are not going to waste money on stronger timbers than they have to.
Secondly, every member of a truss is critical. It is either designed to carry a tensile or compressive stress load. Again, if the timber was not necessary the builder would not put it in
The top boom of the truss ( the bit next to the roof tiles) probably relies on the three internal members for both strength and to reduce the slenderness ratio of the member. It is highly unlikely that this will span from eaves to ridge without intermediate supports
I strongly suggest you ask a structural Engineer to call and give advice, you will undoubtedly need one to provide calculations for your building regs application and though they are not cheap, it can prove far cheaper to take professional advice now than to rebuild or rectify your home later !
David