During the 1990s Montien Boonma became increasingly interested in vessels and alms bowls as stimulants to mystic experience. The drawing Trio depicts three individual vessels that fill the picture plane; the opening for each vessel is created via a simple brush stroke at the top of the work. In his earlier pagoda works, Boonma had already begun to explore the use of materials and working methods to embody the 'real' features of his subject. Each vessel depicted in Trio is thus created out of the perishable materials often contained within: charcoal, earth and ashes. Mixed with glue, the materials used to depict these vessels reveal the exterior and interior simultaneously. For Boonma, a devout Buddhist, this exploration of outer and inner layers was symbolic of his explorations of the body and the soul.

Experiences of the void and death also fuelled much of Boonma's thinking and work at this time: his young wife Chatcham had been diagnosed with breast cancer. Questioning the purpose of life and where souls go after death, Boonma embarked on a rigorous course of study of Buddhist teachings about impermanence and self-reliance. The vessel was used by Boonma as a symbol of impermanence; gazing into it he realised an 'in between' space similar to his ideas of eternity and death. The perishable materials Boonma uses to create his vessels in Trio speak further of an eternal cycle of life, death and rebirth.