Hi Jorge,
yes this can be achieved and this is called u-turning.
you need the below config for this:
1. make sure you have 10.3.0.0 in the split-tunneling acl
2. you need to have traffic from 192.168.15.0/24 to 10.3.0.0/16 in the crypto acl on both the ASA's.
3. you need nat exemption on main office from outside to outside for the anyconnect pool to the remote subnet
for eg nat (outside) 0 access-list nonat
access-list nonat perm ip 192.168.15.0 255.255.255.0 10.3.0.0 255.255.0.0
4. on the branch office, you need nat exemption for the traffic from 10.3.0.0/16 to 192.168.15.0/24