They are all grammatically correct, but there are distinctions worth noting.
Where is specific to spatial positioning, which means you could remove the phrase "in spots."
The cameras need to be mounted where intruders can't reach.
Writer's digest outlines the distinction between "which" and "that" here:
The basic idea is that if the clause that the word is connecting alters the meaning of the sentence, use "that," but if it merely adds extra information, use "which." In your case, the meaning of the phrase is largely lost without the connected clause.
The cameras need to be mounted in spots.
For that reason, "that" is probably preferable over "which," although it is a unique case where I could see arguments made either way.
For these reasons, I would rank the options like this: