Earth signs of Taurus, Virgo and Capricorn are ruled by Venus, Mercury and Saturn respectively.
Air signs of Gemini, Libra and Aquarius are also ruled by Mercury, Venus and saturn respectively.
Mercury, Venus and Saturn are friends of each other. The only bad thing about Earth sign-Air sign combination is combination of undesirable 12th-1st or 4-10th or 6-8th placements which are Dvadasha yoga, Kendra yoga and Shadashtak yoga respectively. However the mutual friendship between the sign owners reduces the intensity of the undesirable yogas.