The thermodynamic equations of the formation of and adsorption at interfaces were derived for a three-phase system having a floating lens with curved interfaces. The surface excess quantities were defined both in the H convention based on the two dividing surfaces and in the P convention based on the four dividing surfaces. Their physical significance and utilities were discussed. The equations were simplified to be applicable to the three-component systems with the plane A/W interface and a relatively large size of lens, such as the air/water/long-chain alcohol systems. The formulation and geometrical consideration of the two conventions with respect to this simple cases revealed that the H convention introduces the distance between the two dividing surfaces and the P convention introduces three kinds of distances between the four dividing surfaces including a kind of thickness of the reference oil lens phase in the A/W interfacial region. Furthermore, the general equations were simplified to be applicable to four-component systems such as the air/oil solution of a surface-active substance/water or the air/oil/aqueous solution of a surfactant with a floating oil lens.