There is no law of trespass in Scotland and you can stay for three nights in any spot. However please be sensible stay away from houses and farmland. If in doubt ask a local as they are sure to know an ideal spot. Try and support local businesses as they are often a lifeline for the local people. On the islands there are often CDP s next to,piers and you can usually get freshwater there and if you are lucky a shower for a pound or so. We have a number of spots we use regularly and they are usually secluded and tucked in wee quiet corners.