Unique or something similar to 3 or more different tables

Question

Sirs,

In the company where I work uses an ERP that I do not know for what reason, is carried out the typing of sales notes, return of sales and returns of purchases in three different forms, and the same activity could be performed in the same form, finally, the nf number field, is equal to the three forms and cannot be repeated, if in the sales form you used nf number 525, the next entry will be 526 in any of the three forms, for this the ERP company used a function that brings the last number used, adds + 1 and finds the next one to be used. Who has a little knowledge, will understand that this is a hell of a gambiarra and that at some point one of the three forms can get the same number, if the typing occurs at the same time. Now, how do we get around that? I already contacted the company about 3 months ago regarding this problem, several charges and I still have the problem here at the company.

Function used:

CREATE FUNCTION [dbo].[NfNumero]

(
    @NfEspecie VARCHAR(3),
    @NfSerie VARCHAR(3)
)

RETURNS NUMERIC(15)

AS

BEGIN

DECLARE @NfNumero AS NUMERIC(15)

SELECT @NfNumero = MAX(A.NfNumero)
  FROM ( 

  SELECT ISNULL(MAX(NfNumero),0) AS NfNumero 
    FROM Venda WITH(NOLOCK)
   WHERE NfEspecie  = @NfEspecie
     AND NfSerie   = @NfSerie

   UNION ALL

  SELECT ISNULL(MAX(NfNumero),0) NfNumero 
    FROM DevolucoesVendas  WITH(NOLOCK)
   WHERE NfEspecie = @NfEspecie
     AND NfSerie   = @NfSerie

   UNION ALL

  SELECT ISNULL(MAX(NfNumero),0) NfNumero  
    FROM DevolucoesCompras WITH(NOLOCK)
   WHERE NfEspecie = @NfEspecie 
     AND Empresa   = @Empresa 
     AND NfSerie   = @NfSerie   ) AS A

RETURN @NfNumero + 1

END