Onkyo TX-DS777 Problem biwiring ohm ?

"Biwiring' is not an accepted term; I suspect you mean 'parallel.'
There are three different ways to connect loads (that is all your speakers are to the amplifier); 
Assume that the speaker outputs are marked + (plus) and - (minus). 
Series - if you connect a + to a - which is then further connected to another plus + which is then connected to another minus, this is a series connecton and if doing this with speakers, each load (let's assume 4 ohms) is additive; in other words 4 + 4 + 4 = a 12 ohm load. The current flowing through each load is identical.
Parallel - In this case, all the + are connected together and then to the + on the speaker connection. All of the - are connected and then also to the minus of the speaker output. In this case, all of the current is shared and not necessarily equally; the lowest load impedance (this is a complex combination of resistance, inductance and capacitance) will draw the most current. If you have all 4 ohm loads, they will all draw the same amount of current. If you have a mix of loads, the final result will be lower than the lowest of the loads.
If a graphical representation is better for understanding, go here:
Series & parallel circuits 
In your case, if the output is specified as (for example) 4 ohms and you parallel two 8 ohm speakers, it will be a 'matched load' and safe for the amplifier. If you add a third speaker of 4 ohms, you will have a load well under 4 ohms, below the rating of your amp, and you risk damaging the amplifier if the output is not adequately protected against mismatches.
For your situation, you should look at the math on the Wikipedia link and avoid causing your amp to fail.