I have 3 VMs at 3 regions. Can I use load balancer to distribute load from users of that region to the VM in that region. I am not able to add back-end pools from different regions.
No, Azure Load balancer does not support distributing the load across the regions.
If you want to distribute the load based on the region, you could use Traffic Manager on the top of your VMs. Traffic Manager provides global DNS load balancing. It looks at incoming DNS requests and responds with a healthy endpoint, in accordance with the routing policy the customer has selected. Read Using load-balancing services in Azure for more details.
For more information, read Tutorial: Configure the geographic traffic routing method using Traffic Manager