I recall some article about the used Tesla market and their cars keeping 90% of their value at 3yrs

. I somehow doubt it.
They are keeping very high rates compared to the rest of the car market. Normally low end brands (and JLR / Aston's for some reason) can be as low as 60% loss ... German Cars generally get 40-50% loss. But a good rule of thumb is 50% every 3 years. So a £60k car in 3 Years costs £30k, and in another 3 years it costs 15k, and after 9 total years it costs £7.5k ... and so forth.
There are always exceptions ... Some are limited editions ... some just become desirable ... some due to high demand and low supply.
Tesla's are one of those rare cases. They are getting as low as 30-40% lose in 3 years, however there is a very big BUT there ... it's very rare to see a used Tesla outside of Tesla. You might find the odd S or X ... but it's very rare. So as all used Tesla's go back to Tesla they can dictate the price. However if people are paying those prices for a used Tesla then they would be mad not to charge that much. Also Tesla want you to buy a new car over a 2nd hand one .. so keeping prices high means you might consider it more as the gap is smaller.
But they are not getting 10% lose in 3 years ... maybe in 1 year. It is also true you can buy a Tesla Model 3 and take delivery, do 1k miles in it ... then sell it for the same cost as a new one - this is because some people don't want to wait 3 months for delivery.
However, in the long term as there are more and more competition to Tesla's and the EV's revolution takes off, used Telsa's won't be in such demand and therefore their depreciation will fall in line with other manufactures - well that's my guess anyway.