As per drodon's reply, I would suggest using a wired link to each cottage.
As they were converted from a single barn, am I right to presume that they are still connected (still a single building, with dividing walls to make them individual units)? If so then laying ethernet lines from where the phone line first enters the site to the other two buildings should be relatively easy either internally (through shared roof space perhaps) or externally (use cable rated for external use and run it in a duct along the bottom of the building - with most building designs you should be able to do this without affecting the aesthetics of the site which may be a significant concern, though you'll probably need to enlist the help of a trained builder to get the cables in and out of the walls cleanly). Lay at least two cables to each of the other cottages, then you don't have to pull everything up in all cases if something goes wrong with one of them (just witch over to the other).
This is going to be easier, though probably more costly, than trying to setup wireless links between the units bridging through those thick stone walls. In the unit where the phone line comes in the minimum you need is the ADSL equipment (and a simple switch if there are not enough ports on the ADSL router), the router can provide wireless for that unit. In the other units all you need is a single wireless AP, assuming that is enough to get good reception around the unit (I would test this before setting anything else up, certainly before buying anything else, by setting up an AP in one unit, connecting a laptop to it and making sure it can stay in contact with the AP where-ever in the unit you move it too, then repeat with the other three).
However you set things up, whatever mix of wired and wireless, there are three things you will need to consider:
1. Make sure that the ISP account you sign up for permits sharing the link. Many, especially those offering "unlimited" or unmetered bandwidth, explicitly do not allow this and would consider the three units to be separate properties which would require their own account. ISP accounts that have explicit bandwidth use policies or simply charge for bandwidth use rather than for the connection (like AAISP
, my current ISP) are much more likely to be fine with connection sharing like this as it doesn't go against their business model (i.e. they are selling bandwidth and related services rather than selling connections effectively irrespective of bandwidth use). It may be that you need to look at more business oriented accounts rather than residential ones for this reason. Unless the terms and conditions explicitly state that sharing your connection is permitted ask the ISP before signing up (please note that I've not checked that AAISP, linked above, allow this so do check before taking my mention of them as a recommendation for your specific circumstances).
2. Bandwidth sharing: as an account that allows the line to be shared between the three properties is going to either have bandwidth caps or be charged per block of data (like AAISP's "units" system of charging for bandwidth use) you may well need to setup at least some monitoring to see who is using what. A small Linux box between the ADSL router and the switch that distributes the network further. One of the small Atom based "nettop" boxes, or even one of the little ARM based "plug computers" (anything with two ethernet ports, or one and a USB port you can plug an network card into), would be perfectly sufficient for this. An ADSL router that you can install custom firmware like DD-WRT on may remove the need for an extra box.
3. Traffic shaping: you may need to protect each unit from each other in terms of one "hogging" the link, especially given you say you are some distance from the exchange so you don't have much bandwidth to start with. Simple outgoing traffic management may be sufficient, to stop outgoing data from one machine throttling the incoming bandwidth for everyone, may be sufficient here though you might want to try some basic incoming traffic management too (incoming traffic management will always be of limited help though, as you can't directly control the remote end of the bottleneck (i.e. the other end of the ADSL line) at all). There are many guides for this online (like this one, for instance: http://www.andybev.com/index.php/Fair_traffic_shapin...
- Google finds many more with a variety of complexity so you should find something suitable for your installation).
If all this sounds far too much hassle, then you may need to consider getting two extra lines and letting each cottage have its own connection, though I presume from your question that you've already looked at this option and found it to be quite expensive to have new lines put in at that location.
Current ISP: Andrews & Arnold (AAISP, aa.net.uk) via FTTC at ~36Mbit down & ~10Mbit up, joined July 2011.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.