As with the dreaded E chord, the 1 and 4 string are sounding exactly the same note, so these are candidates for playing the three string versions, especially if you won't be sitting on them for too long. With the Bb you might find it much easier to play 321x, muting the 1 string with the flesh of your first finger, rather than x211 which still requires a partial bar. With the E you might find it easier to play x442, muting the 4 string with your thumb, rather than 444x which requires you to either squish all your fingers together on the 4 or do a very tricky partial bar muting the 1 string with the flesh of your barring finger. Rock guitars do the latter all the time.
Bottom line - consider 321x for the Bb and x442 for the E.