paucity (n.) late 14c., from Old French paucitÃ© (14c.) and directly from Latin paucitatem (nominative paucitas) fewness, scarcity, a small number, from paucus few, little, from PIE *pau-ko-, suffixed form of root *pau-(1) few, little (source also of few (adj.)).
All three molecules have four pairs of electrons around their central atom. As a result, they all have a tetrahedral arrangment of electron pairs around the central atom. The bond angle in methane, CH4, is the tetrahedral angle, i.e. 109.5 (109 28'). Ammonia, NH3, contains a lone pair of electrons, which repels the bonding electrons to give a bond angle of 107. The atoms have a trigonal pyramidal arrangement.
An example of bent molecular geometry that results from tetrahedral electron pair geometry is H2O. The water molecule is so common that it is wise to just memorize that water is a BENT molecule. The oxygen has 6 valence electrons and thus needs 2 more electrons from 2 hydrogen atoms to complete its octet.