Can I connect multiple WiFi Extender EX3700 to a router?

You can but if you try to use the same ssid, you'll have issues with extenders connecting to each other and not back to the router. Also sometime to keep in mind is that if you're daisy chaining them (router-----ex3700------ex3700----ex3700), every step in the chain adds latency and lowers speeds. 

I usually recommend that if people need more than a single extender or potentially 2 (ex3700----router-----ex3700), that they upgrade to a mesh network.