single-phase transformer

Yes, using two phases of the 3phase system. An example would be a 480/277V 3phase 4 wire Y system. Your single-phase xfmr with a 480V primary would connect to two of the 480/277V phase conductors and your secondary would be a separately derived system of 120/240 1ph3w for example.

No. There is only one primary bushing on a single face transformer. If it has dual bushings than it is designed to use in a bank of transformers. If you connect two phases two it you will be giving it too much voltage and the ratio of voltage in voltage out will be effed up. Meaning the secondary voltage will be high. The voltage between phases is higher than between phase and neutral. Phase to phase = phase to ground x suare of pi

If what you stated was really true, there are a lot of single-phase 480v Pri to 120/240 1ph3w secondary transformer manufactures who would be real upset. The most common transformer connection in the US is derived from a single-phase utility transformer connected to one (your one primary bushing) or two phases of a three-phase system depending on the distribution system, to derive 120/240 single-phase for residential. In small commercial the use of either 480V or 240V primary to 120/240 1ph3w is common. Although when 480/277 3ph4w is used, the 120V is most often derived using three-phase 480 to 208/120 3ph4w Y. I assume the poster's transformer is not a utility transformer and that he/she is working with a three-phase system within a building.