A 3mm wall thickness would be difficult, at best. I would recommend using eddy current testing (depending on material) or digital radiographic testing (RT).
With ERW pipe of this thickness, a variation of eddy current known as EMI (electro-magnetic inspection) is often performed by the producing mill. This could provide a very sensative inspection on steel pipe, but is not applicable to non-magnetic materials. UT inspection services are available, but common practice is to standardize on a notch with a depth of 5% to 10% of the nomial wall (0.15 to 0.3 mm or 0.006" to 0.012"). This is a very shallow notch to pick up with UT, which would make for a lot of false indications.
But, to better determine the best NDT method, you have to better define the types of flaws you are inspecting for.
Yes it is possible, but you will have to precisely specify how it is to be done and what you will be able to detect and measure. The quality of the inspection could be significantly improved by grining root and cap flush.
You will need to use a shallow angle (80 degree?) probe focussed at the right distance for the beam path to be used. Be aware the multiple internal reflections can lead to significant beam spreading.
You will need a reference block with cal and root notches for setting yoour sensitivity.
