I think you have 2 options.
Either you create a table with trigrams and distance adjustment, with '+্+' as the middle character, and also search through this table.
Or you create a table for trigrams, containing only the characters to combine with distance adjustment, and search through it whenever you encounter a '+্+'. In this case you draw the symbol for '+্+' if the adjacent characters are not found in the table.
This will terminate my effort to try to support you, as I have other pending work to do.
Jean-Marc
This forum section might be for you: Jobs and Paid Consultancy