Pretty simple... the bulk of France 450 Mirage III fleet (first flights 1956 - 1958 ) were manufactured before 1964, when C-135FR were procured - and exclusively for the Mirage IVA fleet to reach Moscow. It was a luxury.
First tactical aircraft able to IFR were the F-100s, courtesy of Uncle Sam. Very useful in Africa.
First French aircraft to IFR were the Jaguars and Mirage F1, IOC the same year: 1973.
When Dassault created the Mirage III in the late 50's, IFR was not the norm - for them at least. For USAF it was different of course.
Note that, just like the F-5E, Mirage IIIs were retrofitted with IFR probes... in the 1980's and beyond. The whole bunch of Israeli / South Africa / South America variants: Finger, Panter, Cheetah... had the probes.
That Mirage IIIB trainer for the Mirage IV had its (dry) IFR probe on the nose just because the Mirage IV had its refueling boom, same place.
-Two-seat ? check
-Mirage delta ? check
-IFR probe on the nose ? check
None of the two had a radar in the nose AFAIK, so it helped (unlike the 2000 B/D/N who still had a radar).
Compared to a Mirage 2000, putting an IFR probe on a Mirage III (rounded fat short nose) is kind of PITA as a) the nose is note the same shape and b) the intakes are a bit on the way.
The Mirage V is a bit easier as it essentially passed it main fuselage, nose included, to the Mirage F1 that flew the same year, 1967.
So any Mirage V derivative with the long and pointy nose can borrow the IFR probe of the F1.
Note that Mirage IIIs carries their IFR probe above the air intake when Mirage V have it on the nose, F1 style.
