Routing over multiple Azure VPNs

I have multiple VPN's setup which are not routing properly. I would like to know how I can configure a static route to achieve the following. 

1)Azure vnet Address space 172.32.0.0/21 

2)Azure vnet Address space 172.31.0.0/21 

These two are connected with a vNet to vNet VPN and routing etc is working fine. 

next up 

2) also has a site to site vpn in different subscription to others above . 

3)Azure vnet Address space 172.31.40.0/21 

Routing between 2 & 3 also works fine and I can do all I want. 

My issue is that I cannot route directly from 1) to 3) I therefore believe I need a static route from 1-3 and 3-1 

But I don't know where to add this route.