Question: I have a pair of PMR radios and need a 3rd radio. What would constitute a compatible unit, would it have to be the same make or what?

Stonefisk says:

Well because there are strict rules as to what frequencies to use and how they are divided up into 'channels' there is only a finite number of channels these radios can use. There are 8 centre frequency channels on PMR radios...
1. 446.00625 MHz
2. 446.01875 MHz
3. 446.03125 MHz
4. 446.04375 MHz
5. 446.05625 MHz
6. 446.06875 MHz
7. 446.08125 MHz
8. 446.09375 MHz
All UK PMR radio should be able to use at least 1 of those. With some cheaper units having less than the basic 8. So if you get 8 channel units they will work with all other make and models.
Thing is 8 channel is not many and the service could get crowed so beyond the extreme basic units you get features included of CTCSS tone and/or DCS code. Some sets have CTCSS and some have both CTCSS and DCS.
These features aim to expand on the 8 basic channels. These are pseudo channels, but in reality there are never more then the 8 official allocated channels.
The fact some unit specs boast of many more channels than 8 is because those 8 channels can be used with a coded tone thus a group of radios can paired to ignore all incoming signals except those that are also transmitting the same sub-audible tone. The number of different tones available to use dictates how many of the pseudo channels there are.
The above means that if you want to use the features that expand on the 8 channels then the radio(s) will need CTCSS/DCS. Not totally necessary to go beyond the 8 but you should find most PMR will have at least CTCSS ready to use.
CTCSS = "Continuous Tone Coded Squelch System" aka "Private Line" aka "Quiet Tone" etc
DCS = "Digital Coded Squelch" same as CTCSS but digital, becoming more common to be included in radio spec along with CTCSS.
Typically there are 38 tones to use per basic channel frequency, so that's 304 different combinations for the whole 8 channels, that why some radio boast of hundreds of channels (pseudo channels). Couple that with both CTCSS codes and same again for radio with DCS as well, you get a whole lot more.

Just to illustrate that the basic channel are compatible...
I have a pair of Technika TA-11 PMR radio I use when we are running my dog between two hills on Salisbury plain. These radio cost about 8 quid on offer from Tesco. You would be hard push to find a cheaper low end crappy PMR radio. They have only 3 channels...
1- PMR 1 = 446.00625
2- PMR 4 = 446.04375
3- PMR 7 = 446.08125
Looking at a so called quality PMR radio the Cobra MT725 radio with its 664 possible DCS channel permutation and its 304 possible CTCSS channel permutation it STILL has the basic 8 frequencies, 3 of which of course match what is on my cheap radio.
The matching channels on the MT725 happen to be 1, 4, 7.
1. 446.00625 MHz
4. 446.04375 MHz
7. 446.08125 MHz
So yeah, I can talk with someone with that Cobra and him me on those channels. I might not have same range or audio quality in RX or TX nor any of the other features, but we are compatible and between cars in convoy that's all is needed really.