I wrote something like this
rb 3-($+3) mod 4
as written in FASM manual and I've got an error!

See the FAQ, answer for: "Why do I get "invalid use of symbol" error when assembling PE or object file?".

when i have relocatable pe(dll)
[
virtual
align 4
a = $-$$
end virtual
db a dup 0
]
seems to work

but i still can't see why there is two suggested
align macro variants in manual and only one is suitable
for all cases.

The "align" macro in the section 2.3.3 is just an example macroinstruction and no suggested aligning method, and it is clearly said to be the macro for "data alignment for binary output format".

Thanks a lot