x1 abstractly pertains to an exponential/root/logarithmic relationship between the elements of x2 (ordered pair) which are related via concrete relationship x3.
When discussing the so-called 'Triangle of Power' (see, for example: https://youtu.be/sULa9Lc4pck ), this term is useful, as it briefly encapsulates every possible role which the Triangle may denote/act. Each operator (tenfa, fenfa, and dugri) is a binary operator; x3 specifies which of these operators is being used (modulo SE conversions), and x2 will provide the operands thereof in the order of the inputs given by x3. Notice that lo tenfa, lo fenfa, and lo dugri are all the answers/results of applying the respective operators to an ordered pair of inputs (given by their respective second and third terbri, each); thus, none of these constructs ("lo X") can be submitted to enfa3, which only accepts the operator (not the result of the operator). Either use mekso and mau'au-zai'ai quote the desired VUhU word (te'a, fe'a, or du'o, or their SE conversions), or use a tanru/lujvo in order to create words for "exponentiation operator", "root operator", or "logarithm operator" (or their SE conversions) in order to fill enfa3.