(USGDPPC - EU28GDPPC) / AVERAGE(USGDPPC, EU28GDPPC) = 0.338
So the comment you're replying to was correct, for at least one plausible definition of "1/3 lower than the US".
As for the countries you mentioned:
Monaco: < 1 square mile, not reproducible in a larger country
Norway: Giant oil reserves / tiny population, not reproducible without that
Switzerland: Valid
Ireland: GDP numbers shouldn't be taken at face value because tax laws[2] encourage corporations to attribute EU-wide revenues to Ireland. Reported GDP per capita is 135% of the US value, but 2016 median household income[3] was only 87% of the US value[4]. This cuts both ways, though - other EU countries should have their estimates nudged upwards.
Iceland: 92% of US GDP per capita[1]
Denmark: 91% of US GDP per capita[1]
Sweden: 86% of US GDP per capita[1]
Austria: 91% of US GDP per capita[1]
Finland: 79% of US GDP per capita[1]
UK: 75% of US GDP per capita[1]
France: 74% of US GDP per capita[1]
Italy: 68% of US GDP per capita[1]
Spain: 65% of US GDP per capita[1]
EU (all 28 countries): 71% of US GDP per capita[1]
[1] https://stats.oecd.org/Index.aspx?DataSetCode=PDB_LV
[2] https://en.wikipedia.org/wiki/Double_Irish_arrangement
[3] https://www.cso.ie/en/releasesandpublications/ep/p-gpii/geog...
[4] https://www.census.gov/content/dam/Census/library/publicatio...