Here's why:

* Tetrose: A tetrose is a monosaccharide (simple sugar) with four carbon atoms.

* Functional Isomers: Functional isomers have the same molecular formula but different functional groups.

Let's consider the possible functional groups for a four-carbon sugar:

1. Aldotetrose: A four-carbon sugar with an aldehyde group (CHO) at one end. This is the most common form. Examples include D-erythrose and D-threose.

2. Ketotetrose: A four-carbon sugar with a ketone group (C=O) on the second carbon atom. The only example is D-erythrulose.

Therefore, the three functional isomers of tetrose are:

1. Aldotetrose (2 isomers: D-erythrose and D-threose)

2. Ketotetrose (1 isomer: D-erythrulose)