3 chests is a little less than 600 RP and earns you about 1 random permanent on average. Even less than that if you use essence to perm a semi-random shard from your choice of several random shards since it generally takes more than two disenchanted shards to afford it. Furthermore, your options for random permanents includes champions, which are less valuable than skins; ward skins, which are less valuable than champion skins; and if you choose to get your permanent from a reroll, it's possible you get a champion skin for a champion you don't even own yet.
Compare to a mystery skin. Less RP at just under 500 and will always get you a random skin you don't own and always get you a skin for a champion you do own.
I would say Hextech chests are a bad purchase all-around, even buying keys to use up excess chests (or vice versa if you grind A LOT of keys and ran out of chest drops) doesn't seem like a great deal. If you want a specific skin, buy it directly (you can even wait for it to go on sale if you're patient); if you want a random skin, buy a mystery skin; if you want a champion, just save the IP; if you want a champion faster, just buy it directly with RP (you can also wait for it to go on sale in this case).