Yes, this is a very pedantic question! Is the musical instrument called the triangle badly named? The instrument in question has three sides, but only two angles. Might the name tricosta or triparte be more appropriate?
In some languages the name of the geometric shape for which the instrument is named does in fact mean "three sides" as opposed to "three angles", such as Hebrew (משולש). Therefore the name is in fact appropriate in these languages. This question only refers to the English name of the instrument, without consideration for translations, etymology, or use in other languages.