NEMA 10-30 to European 220V?
A NEMA 10-30 is 2 hots and a grounded neutral conductor, with 110V H1-N and H2-N and 240V H1-H2, all at 60Hz.  Whether an European appliance will accept it depends on the type of appliance:


Appliances with fractional HP motors, or especially AC synchronous (timing) motors, will likely not run correctly due to the frequency difference as motor synchronous RPMs will change as a result.
Grounded (IEC Class I, look for the ground symbol) appliances cannot be safely connected to a NEMA 10 outlet due to the lack of an equipment grounding conductor.  (They can be connected to a NEMA 14 or NEMA 6 outlet with the correct adapter, though, provided that mains frequency is not an issue.)
Some older Class 0 appliances rely on plug polarization and the grounding of the neutral to be safe -- this is common in old consumer audio gear.  These appliances also cannot be safely connected to a US 240V outlet of any type (whether it be a NEMA 6, a NEMA 10, or a NEMA 14) due to the lack of a grounded reference conductor in the 240V section of the circuit.  I'd junk them anyway.
Doubly insulated (IEC Class II, look for the square within a square symbol) appliances (that do not have an AC motor in them) and Class II power supplies (wall warts, laptop bricks, etal) don't care about how the mains is presented to them, and thus can safely be hooked up to a pair of opposite leg hots, even if they are 240V only.  The internal transformer will be built more robustly for 50Hz operation than it needs to be for 60Hz, so the frequency change is a non-issue.
Older instruments and some other types of (older) electronic equipment intended for worldwide circulation will have a jumper link, back-panel switch (the infamous little red switch), or other documented means to change over from 120 to 240V -- this covers older computer power supplies, for instance.  The presence of an IEC-type inlet and detachable (computer type) power cord, instead of a fixed power cord, is a good (albeit not universal) sign to look for a 120-240V changeover function.  Again, the frequency change is not an issue in an all-electronic device.
Newer Class II power supplies and electronic equipment/instrumentation have "universal inputs", and are capable of 120, 208, or 220/240VAC operation without any twiddling -- this capability will be marked on the device's safety label.