Since the launch of the very first foldable phone, durability has been a major concern for would-be buyers. However, Samsung has been refining the design for three generations and has made the Galaxy Z Fold3 and Z Flip3 significantly more durable than their predecessors.
If you want a summary of what was done, this short video from Samsung summarizes all the upgrades. The biggest upgrade is that the third-generation Z phones are the first foldables to have water resistance with an IPX8 rating.
Drop resistance has been improved as in this generation both models use Gorilla Glass Victus along with the new Armor Aluminum. The screen was also redesigned to feature an Ultra Thin Glass and a new protective layer, allowing it to survive 200,000 fold cycles (that’s over 100 cycles a day for 5 years, though note that the tests were done at room temperature).
Besides water, dust getting into the hinge is a concern. Samsung enhanced the sweeper technology that made it better at keeping dust out. Some foreign objects can still make their way into the hinge though, so be careful when using the phone in dusty or grimy environments.