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.