Multivariate Linear Regression based on custom coefficients

I have a quite unique problem. I have a multivariate linear regression problem where my goal is to find the intercept of such regression making sure that the sum of the coefficients is <= 1 and that each coefficient is not negative. I spent a lot of time searching online and I found a great answer here:

https://datascience.stackexchange.com/questions/18258/how-to-force-weights-to-be-non-negative-in-linear-regression

The below code shows how I managed to override the coefficients of the regression using the output from the code shared in the answer above.
My question/issue at this point is: how do I calculate the new intercept value given the custom coefficients?

from sklearn.datasets import load_boston
X, Y = load_boston(return_X_y=True)

from scipy.optimize import minimize
Y = y
# Define the Model
model = lambda b, X: b[0] * X[:,0] + b[1] * X[:,1] + b[2] * X[:,2]

# The objective Function to minimize (least-squares regression)
obj = lambda b, Y, X: np.sum(np.abs(Y-model(b, X))**2)

# Bounds: b[0], b[1], b[2] >= 0
bnds = [(0, None), (0, None), (0, None)]

# Constraint: b[0] + b[1] + b[2] - 1 = 0
cons = [{"type": "eq", "fun": lambda b: b[0]+b[1]+b[2] - 1}]

# Initial guess for b[1], b[2], b[3]:
xinit = np.array([0, 0, 1])

res = minimize(obj, args=(Y, X), x0=xinit, bounds=bnds, constraints=cons)

print(f"b1={res.x[0]}, b2={res.x[1]}, b3={res.x[2]}")

#Save the coefficients for further analysis on goodness of fit

beta1 = res.x[0]

beta2 = res.x[1]

beta3 = res.x[2]   

from sklearn.linear_model import LinearRegression
model2 = LinearRegression(nonnegative=False)

model2.fit(X, Y)
print("Regression intecept =  {}".format(model2.intercept_))
print("Regression coefficient(s) -> n{}".format(model2.coef_))

r_sq_model2 = model2.score(X, y)
print("Regression R-squared = {}".format(r_sq_model2))

model2.coef_ = np.array([ beta1, beta2,  beta3  ])
print("n* Overriden Regression coefficient(s)  -> n{}".format(model2.coef_))    
r_sq_model2 = model2.score(X, y)
print("Regression R-squared with adj coeff(s) = {}".format(r_sq_model2))    

# HOW TO IF I FIND THE NEW INTERCEPT?

Thanks for your help