Not all 555 timers are equal. While the datasheet might say:
Exact equivalent in most cases for SE/NE 555/556 or TLC555/556
This is an example that is not one of the "most cases". Intersil's CMOS 555 timer cannot be used in some circuits that were designed for the bipolar 555. This is one of those circuits. Intersil had to make some compromises in order to get the 7555 to work at a supply voltage of only 2V. The sink and source capacity is one of them.
The parametric table in the datasheet is not of much use here as you are using a supply voltage of 3V. You need to refer to the charts below and even there they only give curves for 2V, 5V and 15V. But the 2V curve is close enough for your purpose. Looking at the sink and source current curves for Vdd = 2V you can see that you never going to sink or source 10ma for your LEDs. And as you try to sink/source current from the output you can see that the output voltage will increase/decrease. As a result it is complicated to work out exactly how much current you are currently supplying to your LEDs but it is obvious that you are sinking much more current than you are sourcing, hence the difference in brightness.
As a result of this the current available to charge/discharge the capacitor also varies greatly between states which is why you don't get the expected 50/50 duty cycle.
The correct way to use this IC in this application would be to use the discharge pin to control the charging/discharging of the capacitor for more consistent timing. The output pin should instead be driving transistors to provide the necessary drive current for your LEDs.
Note: The reason people sometimes design 555 circuits using the output pin to charge/discharge the capacitor is to get a 50/50 duty cycle at high frequencies. At low frequencies, such as in your case, the discharge pin can be used to get close enough to a 50/50 duty cycle that it becomes irrelevant.
Hope this helps.