1. The shop is opposite the bakery.

2. The shop is across from the bakery.
3. The shop is across off the bakery.
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#1 and #2 are okay.
How about #3? Can we substitute 'off' for 'from'? Does #2 mean #3?

1 and 2 are fine, but 3 is not.

No, in this context, #3 is not grammatically correct. The correct phrase is "across from," not "across off." The preposition "from" is used to indicate the direction or location of something in relation to another object or place. So, in this case, #2 means that the shop is located on the other side of the street, directly facing the bakery.