Database
[{_id:1,metadata:{schemes:{_trail:"ABCDEFG"}}},{_id:2,metadata:{schemes:[{_trail:"HIJKLMN"},{_trail:"OPQRSTU"}]}}]
Query
db.collection.aggregate([{"$addFields":{"trail_beginning":{"$cond":{"if":{$eq:[{$type:"$metadata.schemes"},"array"]},"then":{"$map":{"input":"$metadata.schemes._trail","in":{$substr:["$$this",0,3]}}},"else":{$substr:["$metadata.schemes._trail",0,3]}}}}}])