You need ANY three 3-star champions, they do not need to be yordles. You need at least one 3-star yordle plus any two 3-star champions to get 4-star version. That means you will, for example, get Tristana3 + Poppy3 + Maokai3 then you add kled or teemo, and you get Tristana to 4-star. Without being 4 star the yordles are pretty weak.
Currently, 5 yordles are not played, and there are 2 comps that uses 3 yordles. One is with Tristana4 and the second one with Teemo4.
I am not good enough yet to write a guide, but LeDuck has a pretty detailed one - https://sites.google.com/view/tftmeta/s-tier/tristana-carry