You may need to do some research, the Holt being an early crawler, is it complete, does it need hard to find parts, or at this point ones you may have to have built, how rough is it, and the big question, as I can't find a model 50, Fifty, 45, 60 yes, is that the correct model number ?
The other 2 Ten and Twenty are not rare, but not high production either, are those complete, what condition, all of these are more of a collector, interest type, though the previous 2 you may find sales records in recent times, auctions, private sales, but the Holt, could be more of what someone will pay, not a lot to go on here, and you may find the ACME. ACMOC antique caterpillar forums of help with this.
If that Holt is in fact a rare tractor, a recent find or discovery, it may well garner serious interest, but its condition will dictate for sure, parts for all of these may be scarce, more so with the Holt.
Be really interesting to see photos of all 3, but more so of the Holt.
I am in no way knowledgeable enough about Holt, but can recall Ten's in various condition(s) completeness being sold over the years from a few thousand and up.
There were 3 series of the Twenty, "L", "PL", "8C", and the latter was under 700 built, the "L" there was under 2000 built and the "PL" 6300+ built. The Ten "PT series, just under 5000 built. Prior to 12/31, the would be gray, later, yellow, and there is some obscure things to know about some of these within production runs, someone on one of those forums may or will know more.
Hi clearance models are sought after and have high value, they are rare.
An 8C Twenty may have more collector value, lower production but not so low its extremely rare, usually production of under 100 may be considered rare and could be sought after by a collector who may pay a premium etc.
Its difficult to appraise these kinds of tractors without expert knowledge and having all the facts per the above, serial numbers first, condition, scarcity, obscure parts needed, and collector interest will play a significant role in the determination of an approximate value for each.
Tens are nice just due to size, often the side curtains are missing, manifolds, carburetors, magnetos, engine parts, as well as undercarriage components, if needed can change things, as well as originality, side curtains can be fabricated, but if it has the original ones it may have more value. Hopefully some help, sounds like an interesting find or find(s) LOL !