Welcome.
There are potentially a few issues here.
- did you cycle the tank before adding fish, or use an established filter? If you did not, you are now in a fish-in-cycle which is very hard on the fish.
- what are your water parameters (that is, pH and hardness)? Guppies prefer hard water while most tetras prefer soft water. You risk one group not thriving in your parameters. Different fish can also have different temperature needs depending on species.
- if you have both male and female guppies, who are livebearers, you risk being overrun with fry in that size tank. Similarly if they are all females there is a chance they could be pregnant from before in which case you also risk being overrun with fry. If all guppies are males you might see aggression between them (torn fins) as three might be too few to properly disperse aggression.
- what tetra species is it you have? Depending on the type of tetra, they might nip at your guppies. They might also need a bigger size tank to move about. Tetras are shoaling fish and need to be in numbers bigger than three.
- there are many species of pleco, and many grow far too big for a small tank. If your "spotted pleco" is a common pleco, it can in fact grow as large as 60 cm. If you are unsure which species yours is I am sure someone here will identify it from a picture.
