After this card is Special Summoned, if it is sent to the Graveyard, it cannot be Special Summoned with "Monster Reborn".
The "This card is unaffected by other cards' effects." effect is a Continuous Effect. (It is applied while this card is face-up on the Monster Zone. It is unaffected by both players' effects.)
The "Once per battle, if this card attacks or is attacked, during damage calculation (in either player's turn): You can make this card's ATK and DEF become equal to the ATK of the monster on the field with the highest ATK (your choice, if tied). " effect is a Quick Effect. (It does not target. The ATK and DEF gained by this effect remains as long as this card remains face-up on the Monster Zone.)
When resolving this effect, it is not necessary to Special Summon the 3 "Legendary Knight" monsters that were used for the Special Summon of "Timaeus the Knight of Destiny". "Legendary Knight" monsters that were sent from the hand or Deck to the Graveyard without being Special Summoned can also be Special Summoned by this effect.
↑Konami OCG Card Database: When "Timaeus the Knight of Destiny" is destroyed by battle, can you only Special Summon the "Legendary Knight" monsters that were used for the Special Summon of "Timaeus the Knight of Destiny"?