Q:

What cubesats are these, what flags are on them, and why? What's the background story?

This tweet includes the image below and says:

Cubesats deploying from ISS.

Question: What cubesats are these, what flags are on them, and why? What's the background story?

Cropped, magnified and sharpened with abandon:

A:

Top one is Malaysia, middle one Philippines, bottom is Bhutan.
Here's the story in Spacetech Asia's Cubesats from Malaysia, Bhutan & the Philippines on SpaceX’s Dragon launch:

Today, at 5.42 a.m. UTC, SpaceX launched its 15th Cargo Resupply Mission (CRS) to the International Space Station (ISS), with the Falcon 9 lofting the Dragon cargo spacecraft into orbit.
Along with numerous experiments and necessities, Dragon carried three CubeSats belonging to Malaysia, Bhutan and the Philippines. These CubeSats will be delivered to Japan’s ISS module known as Kibo, from where they will be launched into orbit. [...]

Malaysia’s CubeSat launched today was developed by Universiti Teknologi MARA, and is known as UiTMSat-1. UiTMSat-1 is the first nanosatellite developed by a Malaysian university, and has facilitated the setting up of a Center for Satellite Communication within the university.

Also significant is Bhutan-1, the country’s very first satellite. Developed by four Bhutanese engineers, the satellite’s payloads include two cameras and a message relaying system for amateur users from Bhutan’s Ministry of Information and Communications (MoIC).

The third satellite from BIRDS-2 is Maya-1, the first CubeSat from the Philippines, and was developed by two graduate students. Maya-1 will complement Diwata-1, a 50-kg earth observation microsatellite that is part of the PHL-Microsat program implemented by the University of the Philippines Diliman and the Department of Science and Technology (DOST). The two will be joined by Diwata-2, the country’s second microsatellite, slated for launch later this year.

There's a short write-up about Maya-1 in Engineers Rule's Maya-1: The Philippines’ First CubeSat along with a video.

