What is the minimun supply voltage for the pi3b plus.

It sounds like you are measuring at the source rather than on the RPi itself.  You might be having voltage drop in the power supply cable.  You should measure the voltage at the GPIO connector.  If those are voltages are measured at the at the GPIO connector then you need to look at the voltage on a scope to see whether you see lots of ripple or supply droop when running.  It would be good to measure the supply current also.  You could have a bad cable or possibly a bad RPi.  I've seen a lot of bad cables.

Most "standard" USB cables are unsuitable for use as power cable for the Raspberry Pi. They usually use 28 AWG wire for the data signals and 26 AWG wire (lower AWG number is larger

diameter wire) for power although I have also seen 28 AWG used for both. 26 AWG wire for power is way too small for the 2.5 A current needed by the Pi. 22 - 20 AWG should be used

and I like to keep the cables lengths at 4 - 6 ft. (1.2 to 1.8m) max. Most USB cables, in the US at least, have markings that show the wire gauges used. The markings are usually in the form

of 28AWG/2C+26AWG/2C which says that 2 wires are 28 gauge and 2 wires are 26 gauge. The lower AWG wires should always be the ones used for power. You can find USB cables with

22 - 20 AWG power wires, usually specifically described as Pi power cables but you have to look a little harder for them. And of course the descriptions are not always accurate so buying

from trusted sources helps. Being able to visually verify the gauge markings on the cables before purchase is best. I have been able on occasion to use advertisement pictures of the cables

to read these markings but it is usually very difficult to be sure.

Most "standard" USB cables are unsuitable for use as power cable for the Raspberry Pi. They usually use 28 AWG wire for the data signals and 26 AWG wire (lower AWG number is larger

diameter wire) for power although I have also seen 28 AWG used for both. 26 AWG wire for power is way too small for the 2.5 A current needed by the Pi. 22 - 20 AWG should be used

and I like to keep the cables lengths at 4 - 6 ft. (1.2 to 1.8m) max. Most USB cables, in the US at least, have markings that show the wire gauges used. The markings are usually in the form

of 28AWG/2C+26AWG/2C which says that 2 wires are 28 gauge and 2 wires are 26 gauge. The lower AWG wires should always be the ones used for power. You can find USB cables with

22 - 20 AWG power wires, usually specifically described as Pi power cables but you have to look a little harder for them. And of course the descriptions are not always accurate so buying

from trusted sources helps. Being able to visually verify the gauge markings on the cables before purchase is best. I have been able on occasion to use advertisement pictures of the cables

to read these markings but it is usually very difficult to be sure.

Add on top of that the ones that advertise thick wires and still won't work properly