How are 3 digit interstate #'s determined?

[dvferyance]

I always figured they just started at the lowest number (1xx or 2xx) then went up as more were added.  Occasionally a number may be skipped - a planned route never occurred, or an adjoining state already had the low route number so they used the next one to avoid confusion.

Tell that to I-820

Wasn't that State Route 820, then they gave it an upgrade to I-820?  I believe that's the backstory there as to how it was given an unusually high number.

There sure is because radio station WBAP is on AM 820.

[TheStranger]

1.  So basically when choosing which even or odd 1st digit, there is no rule, things vary by state?

From what I've seen in the thread, this seems to be the case.  Some use middle or higher numbers to leave room for lower numbers in the future sequence (i.e. California starting out with 405 and 605 and 805, before creating 205), others do it purely in order (the odd 3dis in South Carolina), and some definitely are more "this number is available, why not use it" (I-820 in Texas stands out, as does I-985 in Georgia).

2.  Why is the DC beltway 495 and the Baltimore 695?

Could this be "Maryland numbering things in geographic order" - 295, 495 both in the DC area, 695 then a bit up north?

3.  If a 3 digit interstate ends at another interstate it must be even, correct, EVEN if that interstate is not an offshoot of 2 digit (Fictionally 387 starts at 87 and ends at 684)?

Not necessarily (see I-355, the Kansas I-135, the under-construction I-369, I-505).  Note that 135 and 505 replaced suffixed Interstate routes (35W and 5W).