The voting system in Puerto Rico is different than that used in US states. It is that difference that allows for smaller parties to gain seats in the legislature.
The voting system in Puerto Rico is a mixed system using plurality-at-large for a fixed number of senate seats, first-past-the-post for a fixed number of house seats, and single non-transferable-vote for at-large seats for both the senate and house. See, Electoral system for Puerto Rico.
The Senate of Puerto Rico currently has 27 members, including 16 members elected from 8 two-seat electoral districts through plurality at large (each Senate district comprises 5 House districts), plus 11 at-large members. The House of Representatives of Puerto Rico currently (2019) has 51 members composed of 40 members elected in single-seat electoral districts using first-past-the-post, plus 11 at-large members.
In each house, 11 at-large members are elected from an island-wide district based on single non-transferable vote. To avoid vote splitting, the two major parties will typically nominate only 6 members and smaller parties typically only nominate one. Additionally, parties may choose the ballot order of its candidates in different districts, in an attempt to signal to voters the preferred method of voting. However, each voter is free to choose any candidate.