No as the current deal is that 3 roams on to Orange in some places only and for 2G only.
3 doesn't operate a 2G network and Orange cannot roam on to 3.
However 3, T-mobile and Orange are all pooling stuff soon as Orange will have access to 3 and T-mobile's masts, Orange will decommission around 7,000 masts of their own and leave 4,000, the rest of their coverage will come from T-mobile and 3.
3 have access to T-mobile masts and 3 masts, but they won't be able to roam to the 4,000 Orange masts as they are mainly for capacity.
Of course 3, and T-mobile they will continue to invest in the network that Orange will be joining.