This is literally the most overpowered mobility champion I have ever seen. Do you even know how long 3000 range is? or 800? 800 is how far Ahri can fling her Orb of Deception, and 3000 is from either wolf camp to the middle turret (or pretty freaking close)
"R. An AD champ with an ultimate like Vi's, but about 3000 range. On arrival, this ultimate should knock all other enemy champions back like Xin Zhao's ultimate. Damage shouldn't be too high, the objective of the ultimate is for focusing the enemy team's well protected ADC in late game team fighting."
Ultimate is insane, just flat out. The way you describe it, it's a suicide run into the enemy in an attempt to divide and conquer, which only a tank could pull off (and no tank has THAT much mobility) There is only one champion that should be able to do that, and 
is squishy as all hell to compensate for his ability to do that.
"E. A dash skill shot with about 850 range (if collides with enemy unit/neutral monster/champion dash will stop short but does damage on collision, like Gragas' charge, but single target) followed by 40% increased movement speed which reduces to normal over 2 seconds."
An 850 unit dash is almost as powerful as a Kha'zix leap with Evolved Wings (900, and he starts it off at 600 units range BEFORE he can leap 900 AFTER an Evolution Point) and the 40% movement speed buff, even if it decays quickly, is still far beyond enough for a chase or engage (which really is insane)
"Q. A ranged slow/stun skill shot like Anivia's Q, but different effect. Has 1sec stun (not effected by CC reduction). After enemy recovers from stun, there's decreased movement speed of 20%, movement speed will gradually recover over 5seconds. NO damage. Needs long range, this is for catching an escaping ranged ADC."
A 1 second stun not affected by CC reduction could be a knockup. This is about the only ability I see that could be implemented.
"W. Should have passive passive bleed damage. Activated should grant 100% critical strike for the duration and give bonus attack speed. 100% critical from level 1. Bonus attack speed is +20/40/60/80/100%. Duration should be about 4 seconds. (like having Sword of the Divine but with an extra second)"
W is literally just way too overpowered with 
assuming this is an AD champion, and is still ridiculous even if not. The extra attack damage in addition would break the game on how high it goes (even 's Shroud of Darkness can, at BEST, reach 80% speed with his shield, which takes practice to block a spell.)
"Champion passive should be 100% movement speed when chasing chasing a target below 20/25/30/35% health."
Even Warwick's blood scent adds even half 100% movement speed even if his Blood Scent is utilized to the maximum potential. 
's Night Hunter (including her Final Hour Ultimate) can do this with 90 movement speed and ONLY WHEN APPROACHING AN ENEMY IN RANGE AND IN SIGHT.
Overall, this Champion would be INSANELY overpowered for it's ability to engage a target, beat them to all hell, and run away like they were never there, which no one should be able to do unless it's a 1v1 in the jungle (which, based on how this character is looking, is NOT what you are aiming for)
Take some time, play a few more champions and try to understand how each one works individually. Try to understand how each ability plays out and why certain abilities have long cooldowns (and why certain abilities have very short cooldowns) The main problem here is that the numbers you are aiming for are too high. I am not saying this champion's concepts can't be done, but you need to tone down the numbers drastically.
but with an extra second)
Wukong (monkey) and 
Warwick (wolf).