Buffers in all three of these coordinate systems will be incorrect (except at a small number of special locations), although the buffers in UTM should not need much correction. Because these are such small buffers (600 m is only 0.0015% of the way around the world), each can be corrected in simple ways and those corrected buffers should closely agree:
In Mercator, buffer by 600*cos(latitude) meters.
In UTM, follow the instructions [in a related thread].
In geographic coordinates, convert meters to degrees (600 meters is 0.00540 degrees) and then expand the widths of the buffers by 1/cos(latitude), keeping their heights (north-south extents) the same. The buffers will look elliptical.
Disagreements may be as great as 0.3% or so of the buffer radius if different datums are used.
These corrections apply, strictly speaking, to buffers of points only. However, such small buffer distances typically are applied only to features of limited extent. Thus, although latitudes may vary throughout a feature, their cosines will likely not vary much: you can use the cosine of a central point to make the corrections. The adjustment in geographic coordinates is sufficiently complicated to carry out, though, that it is best avoided.