transactions, serial ids, and JDBC

I've come to the point where I really need to run a transaction. In the
past it hasn't been as crucial, so I've been happy with individual queries,
but I am now past that point. I am now trying to insert a row into three
separate tables, and the rows refer to each other. Two of them have SERIAL
ids which need to be used as foreign keys. Here's a trimmed down version of
the tables:

CREATE TABLE A (
id SERIAL not null,
somedata int not null,
primary key (id)
);
CREATE TABLE B (
id SERIAL not null,
moredata int not null,
a_id integer not null REFERENCES A(id),
primary key (id)
);
CREATE TABLE C (
b_id integer not null REFERENCES B(id),
yetmoredata int not null,
primary key (b_id)
);

The transaction needs to look something like this:

BEGIN

INSERT INTO A (somedata) VALUES (1);
INSERT INTO B (moredata, a_id) VALUES (1, <id from last insert>);
INSERT INTO C (yetmoredata, b_id) VALUES (1, <id from last insert>);

END

I don't know how to dependably get the id from the last insert. One
possibility, I suppose, is to call nextval myself and use the value
explicitly, but if there is a way to do it portably (i.e. not depending on
PostgreSQL's specific implementation of a self-incrementing id field) I
would prefer it.

Oh, one more thing. I'm doing this from JDBC. Can I do transactions with a
long text string with all of this, or do I need to send each line
(including BEGIN and END) as a separate Statement? Or is there some better
way?

--Greg