Rather using manual typesetting tricks, I suggest you define template boxes for these three notations. Add the three styles below to your document's style definitions. (In case you're not familiar with the style editor, to do this: Format > Edit Stylesheet...; and for each of the 3 styles, start typing to create a new cell, press Ctrl-Shift-E to edit the code, and replace with each cell code shown below, and press Ctrl-Shift-E.)
LeftSuperscript
style:
Cell[StyleData["LeftSuperscript"],
TemplateBoxOptions->{
DisplayFunction->(RowBox[{
SuperscriptBox[" ", #2],
"\[NegativeVeryThinSpace]",
#1
}]&),
Tooltip->Automatic}]
LeftSubscript
style:
Cell[StyleData["LeftSubscript"],
TemplateBoxOptions->{
DisplayFunction->(RowBox[{
SubscriptBox["\[InvisibleSpace]", #2],
"\[NegativeVeryThinSpace]",
#1
}]&),
Tooltip->Automatic}]
LeftSubsuperscript
style:
Cell[StyleData["LeftSubsuperscript"],
TemplateBoxOptions->{
DisplayFunction->(RowBox[{
SubsuperscriptBox["\[InvisibleSpace]", #2, #3],
"\[NegativeVeryThinSpace]",
#1
}]&),
Tooltip->Automatic}]
Input
The boxes themselves are written using box expressions like TemplateBox[{"x","y"}, "LeftSuperscript"]
.
For convenient editing, also create input aliases, by adding another cell to the stylesheet:
Cell[StyleData[All],
InputAliases->{
"l^"->TemplateBox[{"\[SelectionPlaceholder]","\[Placeholder]"},
"LeftSuperscript"],
"l_"->TemplateBox[{"\[SelectionPlaceholder]","\[Placeholder]"},
"LeftSubscript"],
"l_^"->TemplateBox[{"\[SelectionPlaceholder]","\[Placeholder]","\[Placeholder]"},
"LeftSuperscript"]
}]
Then, you can create left superscripts (Esc l
^
Esc), subscripts (Esc l
_
Esc), and subsuperscripts (Esc l
_
^
Esc) while editing using the respective input shortcuts.
Output
To get Mathematica to display the typeset forms in the output, execute
LeftSuperscript /: MakeBoxes[LeftSuperscript[braw_, supraw_], form_] :=
With[{
b = MakeBoxes[braw, form],
sup = MakeBoxes[supraw, form]
},
TemplateBox[{b, sup}, "LeftSuperscript"]
];
LeftSubscript /: MakeBoxes[LeftSubscript[braw_, subraw_], form_] :=
With[{
b = MakeBoxes[braw, form],
sub = MakeBoxes[subraw, form]
},
TemplateBox[{b, sub}, "LeftSubscript"]
];
LeftSubsuperscript /:
MakeBoxes[LeftSubsuperscript[braw_, subraw_, supraw_], form_] :=
With[{
b = MakeBoxes[braw, form],
sub = MakeBoxes[subraw, form],
sup = MakeBoxes[supraw, form]
},
TemplateBox[{b, sub, sup}, "LeftSubsuperscript"]
];
Result
It looks reasonable in both code and typeset-math forms, except for the LeftSubsuperscript
when the subscript and superscript differ greatly in length:

Discussion
Template boxes are harder to mess up while editing than manual typesetting.
Another advantage of using semantically-correct typesetting is that the boxes can be parsed and manipulated as expressions. For example, if you want to use $^y x$ to denote tetration—
Tetration[a_, n_Integer] :=
If[n == 0, 1, a^Tetration[a, n - 1]] /; a > 0 \[And] n >= 0;
— you could define
LeftSuperscript = Tetration;
and use it:

resulting in:

You could also symbolically manipulate or generate expressions containing your notation.