According to the device's page at "https://openwrt.org/toh/xiaomi/mir3g#specific_configuration", it seems to have a single switch controlling all ethernet ports and a single CPU connection; even if you see 2 LAN + 1 WAN ports on the outside, there is indeed one single connection to the CPU. Bear this in mind, as you might not accomplish what you expect (you will never reach higher than 1Gbps connections, for example).
That said, OpenWrt can do link aggregation / channel bonding on ethernet connections, using the standard Linux's support for this feature. However, there is no support for it on uci, so you will have to configure and handle it yourself... nothing that some scripting cannot do.