What is the fruit called that is a cross between a grapefruit,
tangerine, and an orange?

The tangelo is a cross between a tangerine and a grapefruit. The ugli
fruit is a cross between a mandarin orange and a grapefruit. The
temple orange, or tangor, is a cross between a tangerine and an
orange.

Tangelos come in several varieties:

Minneola Tangelo
This is actually a grapefruit-tangerine cross and resembles the
tangerine. The fruit is red-orange and tends to be slightly elongated
and often with some neck.

Nova Tangelo
The fruit is a deep orange color and juicy segments with a sweet
flavor.

Orlando Tangelo
A medium size tangelo which is slightly flattened with a deep orange
rind color.

The Cook's Thesaurus: Citrus Fruits
http://www.foodsubs.com/Fruitcit.html

Dole: Tangerines, Mandarins, Tangelos
http://www.dole.com/health/fruits/tangarines.ghtml

Tony Tantillo: Tangerines
http://www.tonytantillo.com/fruits/tangerines.html

My Google search strategy consisted of "citrus fruits" and variations
of "orange + tangerine + grapefruit"

Best regards,
pinkfreud