Find the assumption, not the motive
It is tempting to read a low price as a warning about the provider. More often it is a difference in what was assumed. One bidder may have priced three attendances where another priced five. One may have excluded internal glass, or left the periodic floor programme out entirely, or assumed you supply liners and hand towel. None of that is dishonest if it is stated, and much of it is stated in a part of the proposal nobody read.
So start with the documents. Line up attendance frequency, hours per visit, the area list and the exclusions across all three. In most comparisons the gap becomes visible within ten minutes. If it does not, the missing information is itself the finding, and the next step is a short written question to each provider rather than a conclusion about which of them is being unrealistic.
