As the previous posters said, I have three lines bonded. I get triple speed in both directions, and I don't have to have multiple downloads, multiple users, multiple TCP connections or anything like that to exploit the multiple speedup. The ISP, Andrews and Arnold does the required dowload magic, and my router handles the upload bandwidth splittting. The lines can be at unequal speeds, indeed one line is about 20% faster upstream than the other two, for some reason. If one line goes down, the system just shifts to rebalance the total load quickly.
I have one IPv4 address and one IPv6 address for the WAN-facing side of the router, and on the lan-facing side, the router presents a single IPv4 and IPv6 address, just as you'd expect. There are not three IPs, one per line, and machines on the internet do not see stuff coming from three lines, so there's no weirdness. It's set up so that a block of sufficient global, routable, static ipv4s are mapped to my lan, and the same for IPv6 (an IPv6 /64 is mapped to the lan).
Costs for copper lines are £12 per month per line for ADSL2, plus line rental, which works out at roughly £20 iirc (VAT ? Can't remember), and the tariff I have chosen is to pay for traffic downloaded by the GB independently which is nothing to do with how many lines there are. I actually pay more than that because I have chosen some extras, including BTW "Premium" traffic prioritisation and faster upload. FTTx would be more per line than ADSL.