John Gill Commentary Isaiah 48:3

"I have declared the former things from of old; yea, they went forth out of my mouth, and I showed them: suddenly I did them, and they came to pass." — Isaiah 48:3 (ASV)

I have declared the former things from the beginning
From the time of their first ancestors, from the time of Abraham their father, to whom was declared what should befall his posterity; that they should sojourn in Egypt, be afflicted there, and come out from there with great substance; that they be brought into the land of Canaan, and the inhabitants of it being driven out before them, (Genesis 15:13–18) . And they went forth out of my mouth, and I showed them ;
they were told to Abraham by word of mouth; they were shown to him in prophecy:

I did them suddenly, and they came to pass ;
for very quickly these things began to take place, even in Abraham's time; for his seed being a stranger in a land not theirs, and afflicted near four hundred years, must be reckoned from the birth of Isaac; and all which exactly came to pass as was foretold; not one thing which the Lord had spoken of failed; all was punctually fulfilled, (Joshua 21:45) .