Currently I have two MS (in stack) and an MX I want to connect a second cable from the second MS is this possible?? because I already tried it and the network becomes unstable, that is, I start losing packets.
With the MS in a stack, they are essentially a single switch. When you connect a second cable to the MX, you're inadvertently causing a loop in the network as you have 2 connections between the devices.
Because the MX does not run spanning-tree, it will pass BPDU's through between the two links. In theory, spanning-tree should detect this loop and disable (AKA spanning-tree block) one of the links.
Have a look at the following thread for more info.
I have 2 pairs of MX with several ports connected in a pair of stacked switches and it has been working without any problem.
LACP & Link Aggregation The MX does not run LACP or any link aggregation protocols. Connecting aggregated ports to the LAN of the MX is not supported; all connected ports should be un-aggregated. If multiple ports are connected to the MX from a single switch for redundancy, it is highly recommended that you run STP on that switch, to ensure that one of the redundant ports is safely put into a blocking state.