adaptation between two pools by messages Exchange ?

Hello;

I have two pools: pool1 and pool2 which exchange data using messages: oru_R01_1, oru_R01_2 and oru_R01_3.
Poo1 sends the three messages on the other hand pool2 must receive them in only one message, I created a third pool: pool3 to make the adaptation between these two pools. Pool3 should receive all three messages and merge it to build a single message. then send this message to poo2.
For this, in pool3 I added three process start signal events, each one will be triggered after a signal sent by pool1. after the reception of the three messages I added an inclusive gateway to merge the messages by a script task after this gateway. During execution, the variables declared in pool three are reinitialized on each reception of messages, which prevents the merging of messages.
how can i avoid variable initialization?
is there another solution to make the adaptation between pool1 and pool2?