Short answer. The three heirs share it among themselves, with each share increased proportionally. Article 852 of the Civil Code provides that if the testator intended the instituted heirs to be sole heirs and their combined shares fall short of the whole, each part is increased proportionally to cover the full estate.
What the law says
If it was the intention of the testator that the instituted heirs should become sole heirs to the whole estate, or the whole free portion, as the case may be, and each of them has been instituted to an aliquot part of the inheritance and their aliquot parts together do not cover the whole inheritance, or the whole free portion, each part shall be increased proportionally.
Civil Code, Article 852 — Shares Increased Proportionally. Read the full provision →
The proportional increase rule
Article 852 of the Civil Code addresses the mirror-image problem to over-allocation: when the testator assigns fractional shares that together fall short of the estate. The article provides: If it was the intention of the testator that the instituted heirs should become sole heirs to the whole estate, or the whole free portion, as the case may be, and each of them has been instituted to an aliquot part of the inheritance and their aliquot parts together do not cover the whole inheritance, or the whole free portion, each part shall be increased proportionally. The uncovered portion does not pass intestate — it stays with the instituted heirs, who absorb it in proportion to their respective shares.
The intent requirement: sole heirs
Article 852's proportional increase rule does not apply automatically whenever shares fall short. The article requires that it was the intention of the testator that the instituted heirs should become sole heirs. If the testator's will makes clear that the three heirs were meant to receive everything — the entire estate, or the entire free portion — then the incomplete allocation is treated as a drafting error. The proportional increase fills the gap. If the will suggests that the uncovered portion was deliberately left out — for a specific legacy to another person, or for intestate distribution — Article 852 does not apply.
How proportional increase works
When the shares are increased proportionally, the relative relationship among the heirs' shares is preserved. If the three heirs were given 40%, 25%, and 15% — totaling 80% — each fraction is divided by the total (0.8) to produce adjusted shares of 50%, 31.25%, and 18.75%. The same heir who was given the largest fraction before the increase ends up with the largest fraction after. The relative ranking among heirs is unchanged; only the absolute percentages are scaled up to fill the whole estate. No new heirs are introduced and no heir benefits more than another relative to the testator's expressed allocation.
Applies to the whole free portion as well
Like its counterpart on over-allocation, Article 852 applies not only when the under-allocation covers the whole inheritance but also when it covers the whole free portion. The free portion is the part of the estate that remains after the compulsory heirs receive their legitimes. If the testator's disposition of the free portion through specific fractional shares to voluntary heirs falls short of the entire free portion, and it was the testator's intent that those voluntary heirs take everything in the free portion, then Article 852 applies to increase each share proportionally within the free portion. The same logic governs: preserve relative proportions, fill the gap, and carry out the testator's evident intent.