When a patient is diagnosed with stage 3 CKD, an eGFR of 38-45 and an estimated loss of 40% of kidney function, can this be reversed with proper treatment?

Once chronic kidney damage has been done, this is not reversible.  If there is a sudden decrease in the estimated glomerular filtration rate (eGFR), then some of this can be reversible.  Once you have reached Stage 3 chronic kidney disease (CKD), it is unlikely this will be reversed.