I watched a lot of these in real time as in they were still pretty current at time of watching The original "The Thing" and "Quatermass and the Pit were too
The book is so much better, a real page turner. Read it in a day on the beach, so had to rely on two Tom Sharpe books to see me through the rest of the week.
Yes only the 3 ... but there is another bad ass Leone / Morricione western that needs a mention, Once Upon A Time In The West... if you have never seen that you are in for a real treat. Epic intro, first film where each major character has their own score, only film where Henry Fonda plays a bad man (and what a performance) and Bronson and that ****ing harmonica! The chimes in For A Few Dollars more are a brilliant touch. Spurly already mentioned The Outlaw Josey Wales ... but going back even older for classic Westerns there is The Searchers ... and even further back High Noon.
The Amicus 'anthology' movies. Dr Terrors' house of horrors, Vault of horror, From beyond the grave etc. And of course Tales from the crypt. AFAIK many of the set stories in those films are based on / derived from actual stories in "Tales from the crypt" and kindred comics. One thing I have noticed over my life with horror/scary movies is that the quality/gravitas of the actors often makes them, not the SFX. The Omen IMHO is a classic example. Gregory Peck + Lee Remick + David Warner + Billie Whitelaw + Patrick Troughton + Leo Mackern. Not "starting out" screen fodder.