I've also seen undervoltage warnings with the Pi 3B + 7" Touch LCD + 2.5A power supply when feeding the power to the display control board and through it to the 3B. This setup has too many connectors and thin wires in series. Also, the dedicated micro USB Y splitter cable from The PiHut/Adafruit causes also undervoltage warnings, so I won't recommend this either.
Simply wire the supply to the micro USB of the Pi 3B, then use jumper wires to power the display. I've got some Polulu empty sockets/housings 2x3 and 1x5 together with precrimped short wires and use them instead of the ones that came with the display, because the stock wires are excessively long.
See also photos here: https://plus.google.com/103555172011502 ... rb3QjC7PeE