As your question is about overall QGIS 3, the answer by MrXsquared is the best.
If you have access to QGIS 3.8 or later, try Add X/Y fields to layer (see visual changelog "New Feature: Add X/Y fields to layer". It can be found in the Processing Toolbox > Vector table).
You can add X/Y fields calculated in the CRS of your choice.
To calculate coordinates of features, open the attribute table of your layer and then the field calculator. Create a new field and make sure to choose the correct field type (E.g. float with precision 8). Enter $x for x-coordinates and $y to calculate the y-coordinate for each feature. Units, by default are specified by the layers CRS.
But of course you ...
Then projection is EPSG:3628.
How to find such info? You try your luck at https://epsg.io/ and search for New York. You get a big list of possible EPSGs and then try.
When you click one, let's say EPSG:3628 (as I did in my first try), you'll see something like:
Don't get discouraged by the text about Puerto Rico, trust the title (New York Long Island), ...
If I understand correctly, you are trying to subtract 1000 from the x coords. Here is how I would go about it:
p1 <- rbind(c(1000,1000), c(1000,2000), c(2000,2000), c(1000,1000))
pol_geom = st_sfc(pol, crs = 4326)
# Getting the coordinates of pol_geom
coords = st_coordinates(pol_geom)
new_coords = coords[,1:2]