  Thanks to the advent of sensitive gravitational wave (GW) and neutrino
detectors, multi-messenger (MM) astronomy will deeply transform our
understanding of the Universe contents and evolution over cosmological times.
To fully exploit the forthcoming GW and neutrino discoveries, it is crucial to
detect as many electromagnetic (EM) counterparts as possible, but up to now,
only one event has been detected by both GW detectors (Ligo/Virgo) and
electromagnetic detectors (Fermi/GBM (Gamma ray Burst Monitor) and Integral),
the short gamma-ray burst GRB 170817A/GW 170817 associated with the merger of a
binary neutron star. To help improving the rate of joint MM events, it is
crucial for the EM detectors in particular at high-energy in space to observe
all the sky with a decent sensitivity. To do so, we propose the development of
3U Transat (TRANsient sky SATellites) project. 3U Transat is a constellation of
nano-satellites offering a full sky coverage with a limited investment. The
goal of this article is to present the 3U Transat project and its main
scientific drivers as well as its current status. We will also describe our
dynamic simulator used to optimise the scientific performances of the
constellation. We will show highlights of the expected performances in term of
detection and localisation capabilities as a function of the number of
satellites in the constellation.
