Hi jNelly. It looks like you're not applying the drop down filter to the original filtered list. You're applying it to the base table. To do what you're doing you'd want to apply the filter to the previously filtered list.
Here's what you're doing.
Filter(InventoryList, available < 3 && available > 0), Filter(InventoryList, StartsWith(Title, Textinput1.text)
What you want to do is Filter(Filter(InventoryList, StartsWith(Title, Textinput1.text), available < 3 && available > 0)
but a better way would be to maybe use the Search function around the filter.
Search(Filter(InventoryList, available < 3 && available > 0), TextInput1.Text, "column1"))