Option 1: This option states that there can be a heat flow between the two systems even if they are at different temperatures. However, this is not correct. In thermal equilibrium, there is no heat flow between the systems, regardless of their temperatures. Heat flow only occurs when there is a temperature difference.

Option 2: This option states that there cannot be a heat flow between the two systems even if they are at different temperatures. This is true in the context of thermal equilibrium. When two systems are in thermal equilibrium, there is no heat flow between them, even if they have different temperatures. This is because they have reached a balance where their temperatures are equalized.

Option 3: This option correctly states that there is no heat flow between the two systems. When two systems are in thermal equilibrium, they have reached a state where there is no net transfer of heat between them. This implies that they have the same temperature.

Option 4: This option suggests that the temperatures of the two systems in thermal equilibrium are slightly different. However, in thermal equilibrium, the systems have the same temperature. Any slight temperature difference would lead to heat flow, which would mean they are not in thermal equilibrium.

Therefore, option 3 is the correct answer as it properly describes the