If you're sold on stick transmitters, you need a 4 channel or more - The left stick moves up/down as normal, but also left/right which is used to control the servo for gear changes.
Three channel sets usually don't have the facility to control 3 speed gearboxes, only 2 speed
left stick to the left and up is first gear/foward
left stick centred and up is second gear/forward
left stick centred and down is second gear/reverse
For some radios you can get hold of a shift-gate, which stops accidental gear shift/selection
The right stick just does steering, although that also moves up/down as well as left/right.
If you get a stick radio with shoulder switches, you can use those to select the gears instead - provided they're 3-position switches.
If you can convert to a pistol radio, you could use a 3rd channel swith for gear change but, again, you need to make sure it's a 3 way switch
For a stick set, take a look at; https://www.modelsport.co.uk/carson-reflex-stick-4-channel-pro-3-1-lcd-with-receiver/rc-car-products/434280
Also, take a look at; https://www.modelsport.co.uk/carson-reflex-ii-6-channel-radio-with-receiver/rc-car-products/396371
The right shoulder switch is 3 way so that could control the gearbox
When looking at stick sets, make sure the left (throttle) stick has a return spring - A lot of 4+ channel sets are for flight and they're not fitted with a return spring by default
The benefit of a spring is if you accidentally knock the throttle stick forward, the truck won't go too far before the stick centres itself
For a pistol set, https://www.modelsport.co.uk/spektrum-stx3-3-channel-2-4-ghz-fhss-radio-system/rc-car-products/444438
"Third channel control with proportional 2, 3 and 4 position controls allows for additional vehicle features like lights, winches and more"
That'll do gear change
"Channel 4 with winch function ( I - 0 - II )"