Frusemide (lasix), carvedilol (coreg), metformin (glucophage), are commonly all used in cardiac patients with diabetes. Rarely there are any problems unless the patient has underlying renal or hepatic impairment which alters the drug metabolism. The most serious as you stated is lactic acidosis. This is at most risk of occuring when the patient has renal impairment with a raise in the creatinine >150mmol/L The only interaction between these 3 medications you have listed is between frusemide and metformin as stated below:
Frusemide >< Metformin hydrochloride - 1 Interaction
Interaction
The generic Frusemide increases toxicity of Metformin hydrochloride
Actions to be taken:
1. avoid combination
2. reduce the dosage of metformin
Drug Interaction Details:
High dose diuretic therapy can cause accumulation of biguanides, thus there is risk of an increase in toxicity. In addition, diuretics may have some hyperglycaemic properties.
Description of classes:
Frusemide belongs to the frusemide and related products class. Sulfonamide derived diuretic acting on the distal loop of Henle, with hypokalaemic effects.
Metformin hydrochloride belongs to the biguanides class. Antidiabetic drugs which increase uptake and utilisation of glucose at the cellular level. These agents do not induce insulin secretion.
Kind Regards
R.
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Lasix-glucophage-coreg 
