You can do something like this. I’m assuming your interfaces are ether1, wlan1, and wlan2.
:global e1tx [/interface get ether1 tx-byte]
:global e1rx [/interface get ether1 rx-byte]
:global w1tx [/interface get wlan1 tx-byte]
:global w1rx [/interface get wlan1 rx-byte]
:global w2tx [/interface get wlan2 tx-byte]
:global w2rx [/interface get wlan2 rx-byte]