There are only three food sources in the game. The alternate food source that was mentioned is probably referring to the replenishment of one of the early food sources - it doesn't stay gone forever.

i've been told that there is only 3 food sources. Once depleated, they do eventually come back.

the berries eventually come back...

So, is it normal that the Berry bush (after i've been farming) only brings in 3 units of food? My farm brings in 7 units. If the berries were only 3 at the beginning, I sure didn't notice it. I fear my villagers are becoming gluttonous.

It's normal! The berry bush continues to provide fewer and fewer berries per basket as your villagers harvest the berries. After the berries are mostly gone, they will get only 2 units of food per basket.

The berry bush brings in different amounts of food depending on how long you forage from it. It goes as low as 2 per trip, I believe.