What Donut417 said,
Load balance with 3 WAN links in pfSense. You'd need some sort of 4G modem still, possible using the router from each provider, or buying new modems, whichever you fancy.
Using load balancing you could see a performance in multi-threaded downloading like torrenting, or when downloading multiple files simultaneously.
Some info can be found here: https://www.cyberciti.biz/faq/howto-configure-dual-wan-load-balance-failover-pfsense-router/