Regarding three-digit interstates: generally if the number starts with an ODD number (105, 305, 505, etc) the route is a spur off of the "parent" interstate. if the number starts with and EVEN number (205, 405, 605, etc) the route is a loop off of the parent road, which will return at some point.

There are exceptions, notably in the SF bay area, where a lack of parent interstates (only Interstate 80) combined with a plethora of interstates, means that the x80 routes don't always follow these rules. Indeed, several don't even intersect I-80!

Regarding route numbers in general: there are exceptions to the rule. The most glaring is Interstate 99 in Pennsylvania, so numbered because the local congressman thought it would be nifty.

Another major "error" is Interstate 238, again in the bay area. As described above, they had a hard time coming up with numbers for the local freeways in the area, so they simply turned what had been CA 238 into I-238. This highway is nowhere near where an Interstate 38 would be, if there were a I-38 in the first place!

There are other exceptions, but these are the most exceptional.