As noted by others you cannot create a 341 by using two vouchers and paying the avios cost for one person. The use of each 241 companion voucher requires the payment of avios for one person with the use of that voucher. To use two 241 companion vouchers you need to pay the avios cost for two people (one for each voucher) and then two people can fly for free as companions. In your case the use of a 241 companion voucher for your 3rd person makes no sense as that person still has to pay the full avios cost.
See these threads (among others):
http://www.flyertalk.com/forum/briti...assengers.html
http://www.flyertalk.com/forum/briti...mex-241-a.html