Hi ronfla
Sorry that you have issues with your BB mattress. 
You are correct a mattress should last much longer than 3 years especially when is using quality and durable materials. BME mattress you purchased in 2015 is now discontinued but it had 2" of Talalay & 2" of Dunlop over a 6" of 1.8 poly support core. and all these materials meet the durability guidelines) You may wish to reach out to BB directly as the mattress has a 10 years warranty and this may be one of those few instances where you’ve received a defective layer. I’d suggest that you measure the depth of the indentation in the “W” shape and take some photos when contacting them. You can read more about warranty guidelines for Brooklyn’s Bedding currently listed in the warranty policy here but the the BME mattress you own has a slightly different warranty that is listed in the warranty policy here.
Since blended Talalay latex is among the most durable foam materials in the industry it will soften much more slowly than any other type of foam so foam softening or breakdown is rarely an issue with latex unless it is defective or too soft for the body type of the person sleeping on it (softer foam is always less durable than firmer foam regardless of the type of foam
In any case … I hope you call them … they have a good customer service and I am sure they’ll look into it and respond to a warranty claim.
Phoenix