my question is how I can increase range of adhoc network using AODV routing protocol?
Short answer: you can't.
The Ad hoc On-Demand Distance Vector routing protocol is just that, a protocol. It doesn't allow more/less dB gain on your antenna.
From the AODV protocol text:
The Ad hoc On-Demand Distance Vector (AODV) routing protocol is intended for use by mobile nodes in an ad hoc network. It offers quick adaptation to dynamic link conditions, low processing and memory overhead, low network utilization, and determines unicast routes to destinations within the ad hoc network. It uses destination sequence numbers to ensure loop freedom at all times (even in the face of anomalous delivery of routing control messages), avoiding problems (such as "counting to infinity") associated with classical distance vector protocols.
In other words, the AODV would allow your 3 RPi's to communicate to any one connected to the MANET which path to take for further communications. It wouldn't increase the connectable range of the devices; on the contrary, one of the points of the AODV protocol is in the event one of your RPi's drops out of range, the other 2 will know how to handle it.
If you want to increase the range of the individual RPi's you'll need more powerful/better antenna's. You can do a lot with software, unfortunately breaking the laws of physics is not one of them yet.
Hope that can add some clarity.