Do only shrimp. Maybe get a male endler. Overall, you're going to either have to cope with too high of a bioload or unhappy fish due to too few in a school. Boraras spp. are jumpers, and need a decent school to feel comfortable. Hell, even with a decent school and a nice-ish lid, they still jump (lost two in the past week - though I haven't found bodies yet, so I'm assuming it was jumping).
Sparkling gourami's are shy and need friends and cover. Tetras are too big for 3 gallons. Badis are also fish that want friends. Killifish - same thing.
I don't know what your plan/capabilities are, but if you make your aquascape have a little height and depth, it'll be great for the shrimp and help disrupt the water column and keep your tank from looking bare in the middle. Maybe get some stones that are a little bigger and build them up with plants around the cracks? The shrimp will hang out on any level, meaning from top to bottom the tank will have creatures.
I also recommend snails, but nerites are notorious for climbing out of rimless tanks. They can survive dry conditions for some time, but I still wouldn't recommend without a lid.