If my opponent attacks my planeswalker (who has 1 loyalty counter) with two creatures (power 1 and 3 respectively), can I absorb the attack of the creature with power 3 with my planeswalker and only take one damage to my health? Or do I treat it like the creature has trample and take three damage to my health?
Neither. Treat your planeswalker as if they are a separate player, one that you can use your creatures to block for. If there is trample damage when your block, that damage goes to your planeswalker, not you. If you don't block (or do block and there's trample) and that damage is more than enough to kill your planeswalker, then your planeswalker dies, and no damage is dealt to you. Damage never tramples over onto you from an attack at your planeswalker. From the comprehensive rules (emphasis mine):
510.1b An unblocked creature assigns its combat damage to the player or planeswalker it's attacking. If it isn't currently attacking anything (if, for example, it was attacking a planeswalker that has left the battlefield), it assigns no combat damage.
Creatures attacking your planeswalker, not you as a player, will only damage said planeswalker (or any blocking creatures). It doesn't matter whether they deal lethal damage to the planeswalker and/or have trample or not. In this particular scenario, the creatures will deal 4 damage to the planeswalker, but 0 to you.
510.1b An unblocked creature assigns its combat damage to the player or planeswalker it's attacking.
702.19b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking.
At the point the attackers are declared your opponent must declare whether the creatures are attacking you or the planeswalker. If they decide to attack the planeswalker with 1/1 creature and you with the 3/3 creature then you will take damage. If they are both attacking the planeswalker, then the planeswalker will take 4 damage, remove 4 loyalty counters from it. As it reaches 0 loyalty counters the SBE will check for this and move it to the graveyard.
Just imagine your opponent attacked a player on the other table. Even if they had trample that damage wouldn't go to you. Planeswalkers had been treated in rules as separate players for years (even though recent changes have changed this slightly) the overall rule still applies.