batch insertion

Hi!
I want quick insert into db a lot of data (in form of triplets). Data is formed dynamical so "COPY" is not suitable.
I tried batch insert like this:

insert into triplets values (1,1,1);
insert into triplets values (1,1,1), (3,2,5), (4,5,5);
...
insert into triplets values (1,1,1), (3,2,5), (4,5,5) .... ;

The more triplets I use the quicker operation is.
With preparation it looks like this:

res = PQprepare(conn, "qu", "insert into triplets values ($1::bigint, $2::bigint, $3::float);",3, NULL);
...
res = PQprepare(conn, "qu", "insert into triplets values ($1::bigint, $2::bigint, $3::float), ($4::bigint, $5::bigint, $6::float), ($7::bigint, $8::bigint, $9::float), ($10::bigint, $11::bigint, $12::float);",12, NULL);
...

The question:
Is there any way to prepare query with any number of triplets without casting such a long string?

Thank you.