Tried before. Mikrotik dont balance well. IF you want to put more than 2 WAN, the modem would not be properly fully used.
then you should try another load balancing approach. 
that’s probably because of metod which MT uses to divide traffic on different gateways - it’s loadbalancing per connection.
Generaly speaking you won’t be able to multiply bandwidth in one stream, but probably using download accelerators (which can use few different sessions to download) or p2p (especialy torrent ones) you should be able to distribute bandwidth among few backbones → multiply download through your internet connections.
But…
one connection will consume 500Kbits, other 30Kbits, so there’s chance that you won’t balance all your backbones well.
Even in mt will try to check upload traffic and balance it regarding to actual load, download is 100% non controlable by you (only ISP can balance your download)
You can make BGP/OSPF, but even if you will distribute upload, you propably won’t balance download…
There are metods to improve this, but i think you doesn’t want to make BGP
The other metod is loadbalancing per packet (vide Mikrotik Bonding)
but the same - you will need to loadbalance on you ISP site too…
In this case you can very accurate divide bandwidth, sometimes clients buying fast connection on some colocation/hosting and pulling two tunells from them to ISP and using bonding they aggregate them remontly
But normaly good enough is classic src-address policy routing
I suggest to use this as “static load balancing” where some hosts like 192.168.1.2-192.168.1.50 will go throug one gw, the other will go through anohter, try to “tune” it using actual load graps and after optimalization, you can use most of your backbones in normal network aplication.