Use a splitter and two different servos and then mechanically hook it up for half deflection operation. You'll need servo wheel quick connectors, pen springs that fit over your pushrods (we used to have these in hardware stores here I don't know where to source them in India) and wheel collars for your pushrods.
The standard bomb drop I use is a servo on one side of the fuse pushing a pushrod out the other side of the fuse. When it's in loaded condition the pushrod extends out of the fuse and when retracted the pushrod sucks flush to the fuse. A rubberband is strapped from one side of the fuse, around the payload and hooked onto the pushrod. In this setup, you would mount your servo mid fuse and servo arm with adjustable connector parallel to the fuse side. Your pushrod for this application doesn't need two wheel collars on it, it could just have an L-bend on one side. Slide the pushrod through the connector, put the spring on and then the wheel collar and then guide the far end through the fuse side. (You probably need to glue a doubler block on the fuse and pre-drill the whole for the pushrod.). Now slide the wheel collar on the shaft til it just touches your spring and lock it down. Pull the switch on your transmitter to drop position so that it retracts the servo as far as it will go. Mark and cut your pushrod for that location. Now, With the switch in neutral you'll have your pushrod sticking outside the fuse to hook your bomb release rubberband on. With the switch pulled the servo will pull the rod in and release the rubberband. With the switch in the far position it will only compress the spring. You will only need to play a bit to find the correct size servo arm to adjust length of travel and the pesky part will be finding the correct springs.
I know that with the above example You don't really need to have a spring to limit the travel, the rubberband will be perfectly happy to absorb the other part of the sawing action, but it will demonstrate how to hook up a control with a Hard stop on one side. I use to use this in conjunction with throttle servos back in the days when servos had one set distance they would travel, the arms were impossible to come by and the throttle arms were itty bitty.
On my Taranis radio though, I'd just assign full spoilers to one position, bomb drop to another and who knows, play music on the third.
I tried to Google the Avionics RCB6i to see what kind of programming is available. I didn't find much, other than basic setup and built-in mixes. I'm sure there must be programming options that I wasn't able to find. The fact that it has a three position switch that isn't dedicated is testament to that. Can you point us to documentation for the RCB6i? I've programmed the three position switch on my 9x, and I'd like to help you out.
What functions or sets of functions do you want to assign to the switch?
Here is a picture of a Dubro Servo saver, a picture is worth way more then a thousand of my words. You don't need to spring load both spring sides of this setup. One side can be a hard stop and just spring load the other side.
Roybro, here is the page for the rx-tx system. If you scroll down, and click on 'download manual', it opens the google reader.
I want it to operate speed brakes and flaps.
When I tested the switch using one servo, the servo arm moved to half throw and then full as i flipped the switches.
Ok, I read through the manual. It says that the 3 position switch isn't programmable. It is configured to move a servo arm to three positions. It is hard wired to channel 5. It can't be reprogrammed to operate functions on different channels.
You could use one servo and have the first position activate the airbrake, the second position further extend the airbrake and release the parachute...
If what you are trying to do is what I am thinking you want to do...
Save up some money and consider buying a Turnigy 9xr or Taranis radio. They are both fully programmable, and have more channels. It's pretty easy to program one of the switches as a safety throttle cut off, and the program the 3-position switch to have different functions based on the position of the safety switch.
I can try to draw something up when I get home. In Minneapolis for work so I don't have access to my stuff I usually use.
I can try to describe it... Twin arm on the servo so you have an arm on either side of the servo. The one side can be used to push the airbrake and the opposite arm can be used to pull a release for the parachute. So, at switch position 0, airbrake is closed and parachute is secured. Switch position 1 has the airbrake half open but parachute is still secured. Switch position 2 opens airbrake and releases parachute. The parachute release can be done like a glider tow release and a spring to kick it out of the container or have a lid it holds closed that releases when the servo hits full deflection.