Hi axio,
I'd assume that you want the 3 installments to be realized in 3 month i.e. 1st installment in 30, the second in 60 and the last in 90 days.

For that purpose, perhaps, you should try setting a payment term first.
1. go to Accounting module > Miscellaneous (scroll to the bottom) > Payment Terms
2. click Create button
3. fill in the Name e.g. 3 installments
4. optionally capture the description of the payment term below the name
5. click Add an item under Computation section. A new screen pops up
6. in the new screen, in Computation drop-down, select Percent
7. enter 0.300000 in Amount to Pay text field
8. enter 30 in Number of Days field
9. click Save and New button
10. repeat steps 6 thru 9, but enter 60 for the num of days
11. repeat steps 6 thru 9, but enter 90 for the num of days, and select Balance in Computation dropdown

Once you've created the payment term you should select it whenever you want to apply it to any invoice.

Hope this helps,
Danijel

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