Anyone else here ever buy a applicator for fertilizer that goes on the end of hose
, mix the appropriate amount of water and dechlorinater in the jug and top
off there pond?

No, but it's a grand idea. It's basically an upside down venturi, that sucks
your application fluid into the jet stream. Quite clever of you.

I am doing it as I write this, I actually put twice the
required amount of dechlorinator

Not necessary, really, but also not harmful.

Don't quote me on this, but I suspect that you don't really need to dechlor
a 200 g top off for a 3000 gallon pond. But hey, better safe than sorry!
Anyway, as I'm admitting to being fuzzy about your amount (10%), why not
google around and see what the net thinks?

at the intake of the pump to spread it around. How do other people deal with
this when you use city water?

Small daily top offs don't really need dechlor, as the chlorine is both dilute
and quickly degasses out of your pond.

If your municipal water district happens to use chloramine (which they may
at any time, without notice, even if they generally don't), the top off will
still evaporate generally, as long as the amount is small.

In aquaria, the zealous often use reverse osmosis (RO) filters on their top
off water. I suspect that a 3000 G pond would put a lot of stress on any
/affordable/ RO unit, though...

C//