spagi wrote:... I get a value of 39ohms. Am I doing this right?
But putting 39Ω in series with it shouldn't hurt.
There's not enough information to make anything but a wild guess, but ...
Blue LEDs have higher forward voltage than other colours, and three in series would have way more than 3-3.5v, so it's likely they're in parallel
If that's right, and it's happy with 3.0~3.5V and 15~20mA, it's possible that increasing the voltage by 0.5v increases the current by 5mA.
That could mean it has a resistance of (0.5/5=) 0.1k, or 100Ω, in series, or it could be that their figures are just estimates, so my calculation is just moonshine
If it does
have 100Ω built in, then putting 39Ω also in series will just make it dimmer.
If it doesn't, then 39Ω in series will save it from distructtion.