No worries, you don't have to justify it. Just buy the 3 if it is better fit for you; it will be for most people. But here are things some prefer about the S:
Considerably quicker in any trim; and P100D available (upcoming 3 P model may match it, but will change the cost comparison)
AWD (also coming for the 3, although it will narrow the price difference)
Twice the cargo space
A large hatch to access that space, and it's powered
More screen space, and one of them in front of the driver
Adjustable air suspension
More range in any trim
Includes a key fob
Center console even with the base model
Dynamic turning lights
Free supercharging generally available
CHAdeMO adapter available
Bioweapon defense mode available
Pano roof available
Rear-facing seats available
I had a reservation for a Model 3 but cancelled it after viewing the back hatch on the initial reveal. Due to the glass roof, the opening into the back hatch is restricted in the Model 3, making placement of large items like golf clubs problematic. In contrast, the hatchback on the Model S is wonderful and allows easy placement and removal of large items.
Although many folks quote a 40K price difference between the Model S and Model 3, the actual price difference will be much less when equipped in a similar fashion. A decked out all wheel drive Model 3 with 275-300 mile range most likely will be pushing 70K and will be a smaller car without unlimited supercharging. I agree the Model S price was stiff, but I never regret the day I used the refund from my Model 3 reservation to help fund my purchase of a 90D Model S. Plus I got the tax refund, a windfall that most likely will not be available when the AWD Model 3 begins delivery.
Saw the 3 for the first time yesterday and was under impressed by its looks. Blended into traffic like the s does not. All a personal preference, but like CraZ8, go for a CPO model s. Best car I ever owned. There is even a p85d listed at $62,300 with only 13K miles!