spend is future tense
spent is past tense
past: spent
The correct past tense for spend is spent.
I've spent 3 years already.
I/You/We/They have spent. He/She/It has spent.
I spend
"Spent" is the past tense of "spend".
Had spent is the past perfect tense of spend.
The past tense of "spend" is "spent" and the past participle is also "spent."
The past simple of "spend" is "spent." For example, "She spent the weekend at the beach."
spent
Spent
Spent