Go, 372 bytes

import(."strings";."strconv")
func d(s string)(k string){for _,l:=range Split(s,"\n"){i,_:=ParseInt(NewReplacer(" ","0","O","1","X","2").Replace(l),3,64)
k+=string('A'+i)}
return ReplaceAll(k,"["," ")}
func e(s string)(k string){q:=ReplaceAll(s," ","[")
for _,r:=range q{k+=NewReplacer("0"," ","1","O","2","X").Replace(FormatInt(int64(r-'A'),3))+"\n"}
return Trim(k,"\n")}

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Explanation

Decoder

import(."strings";."strconv")
func d(s string)(k string){
for _,l:=range Split(s,"\n"){            // for each line...
i,_:=ParseInt(                           // turn the string into an int
    NewReplacer(" ","0","O","1","X","2") // define the replacements
    .Replace(l),                         // replace the chars
    3,64)                                // turn it into trinary
k+=string('A'+i)}                        // append to the output string
return ReplaceAll(k,"["," ")}            // replace `[` with spaces

Encoder

import(."strings";."strconv")
func e(s string)(k string){
q:=ReplaceAll(s," ","[")                // replace `[` with spaces 
for _,r:=range q{                       // for each character...
k+=NewReplacer("0"," ","1","O","2","X") // define the replacements...
    .Replace(                           // and apply onto...
    FormatInt(int64(r-'A'),3))          // the character, turned into trinary
    +"\n"}                              // and add a newline
return Trim(k,"\n")}                    // trim the final newline