[LazyPup]

The first step of your problem would be to find out why your heat vents are not supplying a sufficient amount of heat.

First check your air filter. If the filter is dirty it will reduce the airflow and while there may still be a sufficient flow to maintain heat on the ground floor there may not be sufficient air flow to satisfy the further reaches of the duct system.

Check to make sure that the supply registers or return air grilles are not obstructed by furnishings or personal belongings. Return air grilles are especically critical because you cannot push air into a room unless there is provision for an equal volume or air to leave the room.

It is quite possible that your ducts have balancing dampers which are initially adjusted by the installer to insure a balanced air supply throughout the structure. Once balancing dampers are set they should never be moved but in the real world people often mess with them and get the whole system out of balance which then results in an uneven air flow throughout the structure.

Now in order to understand why you can run three AC's but only two heaters we must consider some mathematical constants.

12,000 BTU = 1 ton of Air Conditioning.
1 Ton of Air Conditioning = 1 Horsepower of Mechanical Energy
1 HP of Mechanical Energy = 746 watts of Electrical Energy

Typically a small room AC is rated at 5,000 to 6,000BTU's or .4 to .5 Tons of AC. Understanding that 12,000BTU = 1Hp we can then say that the AC compressor is a .4 to .5hp motor.

Allowing that 746watts of electrical energy = 1Hp we can then say that the compressor draws .4 to .5 x 746watts or 298 to 373watts.
These small room AC's also have a fan motor which is typically rated at about 1/5HP or 746/5 = 149watts. Thus the combined load of the compressor and fan motor would typically be:

298watts + 149 watts = 447watts
to
373watts + 149watts = 522watts.

Using the formula
P= E x I
Where
P= equals power expressed in WATTS
E= Electromotive Force expressed in Volts
I=inductive Force expressed in AMPS

We can then say that if P= E x I then I = P/E

Typically your supplied voltage (E) = 120Volts
The AC draws approximately 522watts(P) so we can then compute the amp (I) draw for you AC as:

I=P / E
I = 522Watts(P) / 120v(E)
I = 4.35amps.(I)

In order to allow a slight safety margin most small window AC's are rated at 5amps on the data plate.

The small electric room heaters are typically rated at 1,500watts.

using the same formula:

I = P / E we find that a 1500watt room heater draws;

I=P / E
I = 1500 / 120
I = 12.5amps.

The electrical outlets in your bedrooms are computed as "General Lighting" circuits and they are typically supplied by a 15amp breaker.

From this you can easily see that the small room AC's draw approximately 5amps each so theoretically a 15amp circuit could supply 3 of them. However we must also consider that there are other loads on these circuits. From the computatons i would suspect you have at least two breakers for the three bedrooms on the 2nd floor therefore you could run the three AC plus the lamps, clocks, TV's or other small electrical devices common to bedrooms. On the other hand each portable heater requires approximately 83% of the energy supplied by the breaker which explains why you cannot run multiple heaters.