Try 'xsubpp yourfile.xs' and you'll see

XS(XS_REGTEST_rcv_msg)
{
    dXSARGS;
    if (items != 8)
    Perl_croak(aTHX_ "Usage: REGTEST::rcv_msg(query, queryLen, testHos
+t, testPort, prodHost, prodPort, testMsg, prodMsg)");
    {
    char *    int    char *    char *    char *    char *    char *   
+ testMsg;
    char *    prodMsg;
    int    RETVAL;

    RETVAL = rcv_msg(query, queryLen, testHost, testPort, prodHost, pr
+odPort, testMsg, prodMsg);
    }
    XSRETURN(1);
}
[download]

XSRETURN(1) means it returns one. So XSRETURN(3) site:search.cpan.org and you'll find some examples, like

void
GetBorders(handle)
    HWND handle
PREINIT:
    int aBorders [3];
PPCODE:
    if(SendMessage(handle, SB_GETBORDERS, (WPARAM) 0, (LPARAM) aBorder
+s) == TRUE) {
        EXTEND(SP,3);
        XST_mIV(0,aBorders[0]);
        XST_mIV(1,aBorders[1]);
        XST_mIV(2,aBorders[2]);
        XSRETURN(3);
    }
    else {
        XSRETURN_UNDEF;
    }
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or

void
tdxs_wait_for_response()
    PPCODE:
        I32 result = EM_OK;
        I32 sessid = 0;
        I32 token = 0;
        char *cnta = NULL;
        DBCHWAT(&result, cnta, &sessid, &token);
        EXTEND(SP, 3);
        if (result != EM_OK) {
            PUSHs(&PL_sv_undef);
            PUSHs(sv_2mortal(newSViv(result)));
            PUSHs(&PL_sv_undef);
            XSRETURN(3);
        }
        PUSHs(sv_2mortal(newSViv(sessid)));
        PUSHs(sv_2mortal(newSViv(0)));
        PUSHs(&PL_sv_undef);
        XSRETURN(3);
[download]