Overlapping Polyglots
Hexagony, Klein (101) and ???

  xx={puts/}
gets87!@xx=p

main\

&gt;9.*5,6v


This prints 7 in Hexagony, 8 in Klein (101), and 9 in ???.

The 9 does not work if a newline is added at the end of the code. Be careful if you're testing locally.

Edit: Being live for 20 hours is already a record, so I'll give some hints from now on. (Also because the language in question is IMO not yet well-known.)

Hints


"The 9 does not work if a newline is added at the end" is very significant, as well as the first two spaces (which are ignored by both Hexagony and Klein).
The language is on TIO.
The first two spaces make the program jump to the last line. (It's not a 2D language.)
There's no explicit output command, and the v command ends the program.


Explanation (cracked post)

In the source code

abcd={} -- a gaffe avoiding tactic in C++/C#
abcd[!1]=   1+2+3+4+5+6+7+8+9+10+11+12+13+17!
print(abcd[0&gt;1+2+3 and 4+8+6&lt;0-0]//35)
if 0&gt;1 then a.next=';' en\
                    &gt;1+6.@.@


The relevant instructions in Klein (101) are:

IP-&gt;.....................^...........IP-&gt;/
.........................|...................
.........................8
.........................\&lt;-.............IP&lt;-
                         @


Klein is fungelike 2D language where crossing the boundary of code area (which is a square) depends on the Klein topology. / and \ are mirrors.

The IP starts at upper left corner facing right. It meets a mirror towards the top boundary, and re-enters the code area on the right side as shown above. Then it hits the mirror again, 8 is pushed, and then (after passing through the boundary several times) stops at @. Then the content of the stack is printed to stdout, which is single 8.