I have three seperate computers, which I currently do not have an OS on. I also have three seperate internet connections with connection speeds of 256K, 512K and 1M. I would like to use all three of them to download a large file concurrently - How would I do this?
There is no simple (ie practical) way to do this.
As Journeyman Geek said, if its a torrent, then definitely thats the way to go, however you will still need to sort out your routing so as to balance the 3 connections - something which is not trivial.
If it is NOT a torrent, either with the help of your ISP or using VPN's if your ISP won't help you and you have an external endpoint you can use for this purpose with adequate bandwidth, you could probably set up multiple VPN's and do some kind of weighted round robin routing to send the packets across the 3 links - although this has issues of its own (like out-of-order packets caused by different paths and latencies).