I actually weighed the stock TR steel rims against the wire wheel variants once. The stock pressed steel TR rim from a TR3 was the lightest (something around 14 pounds), followed in turn by the 48-spoke, 60-spoke, and 72-spoke wheels. The 72s that I weighed were Morgan SS wheels (which look great, have the right offset, and go right on an early TR like they were made for it). TR-6 wheels would be even heavier. The 48-spoke was the only wheel that was at all close to the disc wheel but has flimsiness-issues.
The funny thing is that probably most of the "lightweight" wheel replacements (Panasport, et al) are probably heavier than the regular TR disc wheel - though they'll carry a wider tire. As time goes along I've been appreciating the stock TR3 disc wheels more and more. When every early car has been restored with alloy wheels or wires, the stock wheels will be the rare thing to see at a show or event.
Light wheels are a major fetish in the early-Miata world (have one as my daily beater, and run 14" BBS there) and make quite a difference to handling with the Miata's suspension design, but I've never been sure about on a TR3.
Would enjoy hearing from others on their experience with overall tire+wheel weight on a TR