The concept of bundling the three long life deities, Amitayus, White Tara and Ushnishavijaya, together as a set is a Tibetan construct. The three are the most popular and most represented longevity deities in Tantric Buddhism and art. (See the Health, Healing & Longevity Page).
"Bhagavan Lord of Limitless Life and Pristine Awareness with a body red in colour, one face, two hands and with two long eyes glancing with compassion on beings, gazing on the entirety of migrators; and a smiling face, wearing the complete sambhogakaya vestments. Above the two hands held in meditation is a long-life vase filled with the nectar of immortality; with the hair in tufts, adorned with silks and jewels, seated in vajra posture, the body blazing with the shining light of the  marks and  examples." (Sakya Tridzin Kunga Tashi, 1656-1711).
"...with a hue white like an autumn moon - radiant like a stainless crystal jewel, shining with rays of light, one face, two hands, and having three eyes; with the conduct of having sixteen years of age. The right hand is in the mudra of supreme generosity; the left holds with the thumb and forefinger the stem of a white utpala to the heart with the petals blossoming at the ear. Representing the buddhas of the three times the single stem is divided into three, in the middle is a blossoming flower, the right in fruition, the left in the form of a bud; adorned with various jewel ornaments; having various silk upper garments and a lower garment of red silk; seated with the legs in vajra posture. The palms of the hands and feet each have an eye - the seven eyes of pristine awareness." (Sachen Ngagwang Kunga Lodro, 1729-1783).
"...Ushnishavijaya, the colour of an autumn moon, with three faces, white, yellow and blue and eight hands. Each face has three very large eyes. The first right hand holds a vishvavajra, second a white lotus with Amitabha [Buddha] residing, third an arrow and the fourth in [the gesture of] supreme generosity. The first left holds a vajra lasso, second a bow, third [in the gesture of] bestowing protection and fourth in [the gesture of] meditative equipoise holding an auspicious nectar vase; complete with silks and jewel ornaments, seated in [vajra] posture. Within the outer circle of the stupa, on the right [side of the chaitya], above a moon is Avalokiteshvara with a body white in colour, the left hand holds a lotus. On the left [of the chaitya], above a sun is Vajrapani, blue, the left hand holds an utpala with a vajra; standing in a peaceful manner and adorned with silks and jewels." (Jamyang Kyentse Wangpo, 1820-1892).
The images below are only a selection of examples depicting the Three Long Life Deities (tse lha nam sum), Amitayus, White Tara and Ushnishavijaya.
Jeff Watt [updated 6-2021]
(The images below are only a selection of examples).