You get this capability in numerous Motor Protection Relays (MPRs) and now some Solid State overload Relays like the A-B E300 by adding analog input modules, so you can kill 2 birds with one stone.
But there are a few RTD monitors out there that just look at the RTDs, most of them are from smaller lesser known entities so their long-term support is something to consider.
TECSystem out of Italy is one of them, I've used them numerous times. The relay is however a "multiplexer" (MUX) for up to 8 RTD channels, so it has an annoying "click-click-click" sound as it changes from one RTD to the next to the next ad nauseum.
Motortronics from Florida makes one as well that monitors up to 12 RTDs (parallel monitoring, not a MUX). I've used it too and IIRC it was less expensive than the TECSystems one, but their tech support is iffy sometimes. On that project I didn't do the commissioning in the field, so I don't know how well they worked, then the entire project was mothballed a year later, so I can't attest to the long term success. But they did at least outlast our warranty...
Given the vagueness of the specification (unless you aren't telling us everything), you could also go the route of just looking for a Multi-Channel Temperature Controller, as would be used in process controls, and find one that accepts at least 3 input channels and accepts RTDs. Places like Omega that sell instrumentation often have web guides that you can use to pick features you need and they will steer you to the right product. The thing with these is that they are not typically configured for motor protection, which usually includes the need for features like "voting" in which the relay doesn't trip unless at least two of the inputs agree (helps prevent false tripping from bad sensors). Both of the motor RTD protection relays above have voting as an option. But if they are only wanting 3 RTDs, I don't think they intend on requiring voting.
