isn't that why they are in multiples of 5? 0,5,10,15,20? 5x3=15? 15W is 3x a thick at 5W?

Nope.

The number before the W is just the Winter rating for the lubricant.

The Winter rating is based on two parameters:

1. CCS - Cold Cranking Simulator - this stipulates a viscosity ceiling for a given rating to minimize the impact on cranking speed

2. MRV - Mini Rotary Viscometer - this is another viscosity ceiling but designed to confirm the oil is able to be pumped

CCS and MRV are 5C apart and are measured at the following temperatures for each respective Winter rating:

0W-xx: -35C/-40C

5W-xx: -30C/-35C

10W-xx: -25C/-30C

15W-xx: -20C/-25C

20W-xx: -15C/-20C

So, for example, on the CCS test a 0w-30 could be 4,800cP @ -35C and a 10W-30 could be 4,500cP @ -25C, at which temp the 0w-30 would be ~1,200cP. But both oils could be close to the same visc at 0C, heck the 10w-30 could actually be thinner if it's a GF-5 lube and the 0w-30 is a Euro lube.