In experience to get a proper working multiple wan configuration using mwan3 starting from scratch you should:
Important: this works well on OpenWRT 15.05.1, on newer versions there was some breaking changes, for example, the wan ifaces have ipv6 capability and now are named with letters ("wan, wanb... , wanc" instead of "wan, wan2... wan3" so wanb6 means 2nd wan ipv6.): https://github.com/openwrt/packages/blob/master/net/mwan3/files/etc/config/mwan3
The official documentation seems to be very detailed and up to date, I recommend reading those first: https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3 but I recommend to give a look at my config file below, since my approach for policyes is very nice.
First of all: Activate conntrack, docs says that is important and neccesary to get MWAN3 work properly, and it is needed to reboot: