Peal and slice 4-6 cucumbers. Cut the cucumbers into fourths lengthwise before cutting into slices. You will get 3-4 cups of cucumber slices.
Next cut 2-3 medium tomatoes into thin wedges.
Slice one small to medium red onion into rings about 1/4 inch thick. You can cut the rings in half.
Put all the vegetables into a large bowl or crock. They will marinate in this at least overnight.
To make the marinade, in a pint mason jar with lid mix the olive oil, red wine vinegar, salt, and sugar. Put the lid on the jar and shake until mixed. Poor the marinade over the vegetables and over with plastic wrap.
Let the vegetables marinate overnight in the refrigerator.