As Bill has mentioned, all link in a LAG (or LACP) is active.
But there are some options you can configure in sharing configuration. According to "algorithm" configured to load-balance traffic among the ports and traffic type in the network, the ratio of loaded traffic among the ports can be different. In other words, all traffic can be passed through port 1:2 only (no traffic on port 1:3 even though it is UP) like you said.
Please refer the "Link Aggregation" section in EXOS User Guide regarding the various options:
http://documentation.extremenetworks.com/exos_22.2/EXOS_21_1/Slots_and_Ports/c_link-aggregation-on-t...