15 miles of actual mileage used daily, which is mostly short trips (most likely) will be between 20 and 25 miles of actual range used. The car may have a EPA estimate range of "358 miles" but:
1. You dont charge to 100% all the time (at least not on long range vehicles currently). You will charge to at a max daily of 90%. 90% of 358 is 322.
2. You dont run down to Zero (no one is doing that on an EV). Generally, people would be looking to "fill up" when they hit 20% (approx 71 miles)
322 - 71 = 251 miles. This is the number that someone will be using (90% to 20%).
15 miles driven, given weather and short trips etc will be 20-25 miles used from the cars "range meter". This does not account for sentry mode ( approximately 1 mile an hour of usage) or climate control. People without home charging like to use sentry mode because the car is generally parked outside, but in this specific OPs case its not apparent whether this will be desired, so lets discount Sentry mode usage.
Call the "15 miles driven" 22 miles used, and tack on an additional 6-7 miles for climate use etc and call it total of 27 miles used. That gives 9 days to go between 90% and 20%, not counting on the fact that winter usage is less efficient (about 30%). In actual usage, this OP will be going from 90% to 20% in somewhere between 7-9 days, certainly not 2 weeks, and no where near 3 weeks.
They will also be spending a fair amount of time at a supercharger to charge back up to 90%, due to how the charging speed tapers while supercharging once the battery passes about 60% or so.
This is all when brand new, not counting at all the average 5-7% battery degradation that occurs the first year or so, before it appears to settle down.
I am certainly not down on teslas or model 3s, but really try hard to educate people who dont have (or think they need) home charging because they are going to "fill up every couple weeks" that it wont work like that, so they either need to be prepared for what it ACTUALLY will be, or not get the car.