Well then
!
I won't delve into trying to help you choose a particular body, there are various very good options available and you'll get all kinds of recommendations.
As to the 1D3, well, it comes from the line of "professional" 1D bodies, meaning it is designed to deliver high performance and comes with a rugged build meant to "resist" the elements as well as take a bit more of a "beating".
What this all means is that it serves well as a "workhorse" for either pros involved in things such as sports shooting, event shooting, photojournalism, fields where you need the ruggedness and the high performance and overall reliablility. It delivers a high frame rate, 10 frames-per-second, which means a lot for certain types of shooting. It has a sophisticated Auto Focus system that can be "fine tuned" for shoots that demand not just a fast AF but also one that can be configured for a variety of shooting challenges. Also, it has a built-in "grip" that carries a large, long-lasting battery -- you can shoot, well, a lot of shots off of one charge, so for instance a sports shooter can fire away without worry.
And then, there is the "serious amateur" like me, who will also be shooting demanding things. Common areas are wildlife/birds, sports, street shooting, events, many times overlapping the pros, and for many the 1D3 is also a "camera of choice. Anyone engaged in outdoor shooting a lot can appreciate the weather resistance and the ruggedness, and the same applies to the performance features.
Also, the image quality is very good, including a good high ISO/low noise performance. I've never shot a 50D, but I know of folks here who have found the need for better high ISO performance, and the 1D3 would deliver well.
But, it's heavy -- the "main" body is bigger/heavier than the 30D even without the built-in grip, so the whole thing is considerably bigger and heavier than the 30D/40D/50D. For some people, it's too much. In fact, for my day-to-day shooting, I tend to favor one of my other bodies that is lighter/more compact. I also have a 30D, by the way, although my "go to" camera for a walkabout is the 5D "Classic" -- bigger than the 30D but still less big and heavy than the 1D3 (the 1D3 is about as big as a 5D with an added grip).
Like I said, there are numerous good offerings out there! So really, it's up to you to figure out where you want to go with your photography and so what do you really want in a camera, and then factor in the various features of the various bodies available.
And, by the way, as far as a higher performance very capable body goes, you could consider the Canon 7D. It has newer features than the 1D3 and also has many of the same Auto Focus "strengths". It is said to have quite good IQ and can be used at a very high ISO.
And, it's a smaller, lighter body than the 1D3. It's more expensive than the 50D or 60D, so take your time in deciding, like I said, what you really want/need to carry out your photographic aspirations!