A suicide bomber blew up a truck loaded with explosives today at a former monastery where six Lebanese Christian leaders were meeting to discuss their opposition to a Syrian-sponsored political reform plan for Lebanon. The bomber and three other persons were killed and 26 injured, including several politicians.

The explosion, at Aukar, a Christian suburb of east Beirut, demolished a wing of a former monastery and broke windows 300 yards away at the U.S. Embassy annex, the site of a suicide car bombing in September 1984. No injuries were reported there today.

In addition to the unidentified driver of the truck carrying the explosives, two Lebanese Army guards and a woman outside the building were killed. Authorities said the thick walls of the building prevented serious injury to the Christian leaders meeting inside.

Those hurt included former president Camille Chamoun, 85, who emerged with minor cuts and bruises. Four others -- George Skaff, a former defense minister; Elie Karameh, president of the Christian Phalangist Party, and two members of the Christian Lebanese Front, Eduard Honein and historian Fuad Ephrem Boustany -- were hospitalized with wounds.

Callers claimed responsibility for the bombing on behalf of two previously unknown groups, but authorities said it was impossible to verify the claims.

One caller, saying he spoke for the Free Christian Youth Organization, said that "the era of opportunists placing their personal interests above everything else and those Christians in the hands of Israel and Syria is over." Another claimed responsibility on behalf of the Vanguards of Arab Christians, saying, "This is the end of those who turned to Israel and of the enemies of Arabism."

The apparent assassination attempt against the Christians, who represent the traditional leadership of Lebanon's divided Christian community, coincided with growing Moslem and Syrian impatience with their reluctance to sign a draft accord for political reforms under Syria's sponsorship.

The draft accord, kept under wraps by Syrian leaders and Lebanon's three main warring factions -- the Christian militias, the Shiite Moslem Amal movement and the Druze Progressive Socialist Party -- reportedly would redefine and limit the powers of the president, who traditionally has been a Christian under Lebanon's 42-year-old power-sharing arrangement. It calls for expansion of Lebanon's parliament to increase the Moslem representation, making it equal to that of the Christians, and for formation of an inner cabinet that would decide major constitutional changes by consensus.

The accord, aimed at ending Lebanon's 10-year-old civil war, would confine the Lebanese Army to its barracks for rehabilitation and eventual use against Israel, and expand the role of a paramilitary police force, using factional militias. It emphasizes political and military cooperation with Syria.

In broad terms, the draft accord is aimed at paring down the Christians' monopoly on top state positions and at providing the Moslem community with a greater share in decision-making. Fears among Christians that such blanket reforms would endanger their community and lessen their power have blocked progress toward Syria's national reconciliation project.

The joint Christian militia, the Lebanese Forces appeared initially to accept Syrian and Moslem demands for radical poltical revisions. But it refused to sign the accord as expected on Nov. 3, expressed reservations to Syria and asked for more time for consultation within the Christian community.

The Lebanese Forces and the Lebanese Front, which called the meeting that was attacked today, disagree with policies of Lebanese President Amin Gemayel and his Christian-based Phalangist Party.

Nonetheless, the Lebanese Forces' executive committee, which has been accused by rivals of selling out to Syria in order to upstage Gemayel's party, condemned today's bombing.

Moslem leaders have expressed impatience with the Lebanese Forces and charged that the Christian militias were reneging on their commitments.

In Damascus, the state-run press warned those "playing with fire" in Lebanon and it said no one could blackmail Syria or hinder it in its efforts to "stand by Lebanon."

Today's attack left a 20-foot-wide crater in front of the former St. George's Monastery, which had been turned over to the Lebanese Front several years ago.

Lebanese Army soldiers fired at the truck as it sped into the compound. It exploded before reaching the building.