I was just wondering if it is acceptable to justify my choice of case studies for my PhD on the basis that I don't have the resources to travel extensively. I have a choice of around 8 locations, all of which would be suitable for what I'm trying to do. I can narrow those 8 down to 3 on the basis of added value in terms of the type of data I would get. I would love to look at all three but that's not going to be possible. Of those, two are on the other side of the Atlantic while one is in Europe. I'm leaning towards the European one because I'll be making short week/fortnight long trips once a month and need to make every hour count. Going to Europe means i) cheaper travel, ii) no jet lag. Beyond that I'm really struggling to think of reasons why I should go there and not to one of the destinations across the pond.

Any thoughts/experiences with this kind of decision making?

Can you justify it as a convenience sample? Can't see why you shouldn't be able to use that approach as a valid reason.

Forgive me if I'm being daft but if all three sites are basically equally good and one site is enough (i.e. not the bare minimum but providing an appropriate scope for a doctoral study) then do you have to justify your choice any further than:
"There are eight possible sites. Of the eight sites, three possess these particularly interesting factors. This thesis will focus in site A. There would be value in testing the conclusions reached in this thesis in relation to sites B and C but that is not within the scope of this thesis (see directions for further research in the thesis conclusion)."
Am I missing the point?
I think this is sort of similar to a choice I have made whereby there is a big body of work produced by one author and I'm only looking at a specific set of works - not because the others aren't interesting or valuable, but just because a thesis needs to have a focus which is an appropriate size so that the material can be tackled at the right level of detail.
Does this help?

I'm with the others - you can justify it on grounds of ease of access.