This is a really interesting but difficult question, because no matter what, I'd sacrifice something!
I don't know if you mean product categories or also specific brands, so I'm just going to mention both.
It would probably be this though: a cleanser, a retinoid, and sunscreen.
I don't think I could do without cleansing, because it's the fundamental basis of my routine. Yeah, it's boring and doesn't seem to do much on the surface, but removing dead skin, dirt, oil, and applied products is necessary. My skin also feels really good after cleansing and I don't want to imagine weeks worth of greasy sunscreen build-up on my face. The cleanser I'm currently using is the Neutrogena Hydro Boost Aqua Gel – I love its surfactant system, how it feels, and the fact that it comes in a pump!
Some kind of retinoid, preferably tretinoin, for sure, simply because of their proven "anti-aging" benefits. Also for acne, I guess.
Sunscreen (with antioxidants) would be another must, mainly to reduce photo-aging and skin cancer risk. Currently using La Roche Posay UVMune 400 because of new the Mexoryl 400 filter. It's the best sunscreen I've used, not only in terms of protection but also due to how it spreads and how non-greasy it feels on my skin. Personally, it's enough moisturization for me.
With this setup, I'm missing out on exfoliation, so if I could have the chance to NEVER LEAVE THE HOUSE, I'd pick some AHA/BHA instead of sunscreen and never see the light of day, haha.
Trilogy Rosehip Transformation cleansing oil, Bloomeffects royal tulip nectar moisturising nectar and Bondi Sands Hydra UV protect SPF 50+ face fluid. It's hard to choose between Tretinoin and the Bloomeffects moisturiser, especially since I think the tretinoin has more of an overall effect on my skin but for the times i need moisturiser I REALLY need moisturiser. If I were allowed to have the moisturiser as a back up item that was hidden away in my back up cabinet I would choose Tret instead as part of my 3 :D
Kiehl's Ultra Facial Moisturizer with SPF 30 (i wear it every day)
The Ordinary Niacinamide Serum (keeps acne, hyperpigmentation, and.fine lines at bay)
Vaseline (slug life)
If I can buy bodycare items separately I'll be alright - I'll slip in something I can also use to wash and exfoliate my face (e.g., Dove unscented beauty bar).