I have a similar setup on my Taranis. I have the basic gimbal tilt set to a slider on the side. Then I have a 3 position switch set to override the slider value when flipped. Switch up is set to no override, center switch is straight ahead with a slight down tilt to avoid the rotors and down switch is pointing straight down. I just created a new mix for channel 5 and set the "Source" to the switch I wanted to use. Then set the "Switch" to the desired position and used the "Offset" to tilt the gimbal to the desired position. Make sure you have "Multpx" set to "Replace" so it will override your slider. Repeat for the next switch position and offset.
This setup will only allow you to have two positions stored. If you programmed a tilt for all 3 positions it would completely override your slider. You could, however, setup a toggle on another switch that activated the override for the 3 way switch. Maybe with a custom function/switch? There are many ways to accomplish what you want.
Here is a link to a video that might give you ideas on how to proceed.
https://www.youtube.com/watch?v=-gA5o_g ... I8zIW_ecbg