is it a good idea if bi couple seeking a third to begin or life?

No, everyone is different, and yes they do require another level of personal integrity, I agree. I am also bi and I have successfully integrated a partner into a previously long-term monogamous relationship before in the past. What you must be fully aware of however is the ability for both you and your current partner to have clear understanding of yourselves, reactions, and boundaries when integrating a third member. You may know your partner's quirks are triggers, but a new person will not. You cannot speak for your partner unless given permission to, and all members must be prepared to always listen first and speak second with clearness, always making sure that things are known by each member the same way. Although you and your more experienced partner may work as a team, you also have to remember that you can't make plans to bring someone under your wing without their consent as well; treat every member as individuals who work together on an equal playing field. These things are important to communicate even if you're only pursuing the others person sexually and they are not tied to you romantically or emotionally. Always be upfront and honest without acting on emotional blindness when communicating. For example, if you get jealous at any point, tell your partner(s) that you feel jealous without acting out on the emotion. Everyone should feel prepared to give reassurance and comfort whilst acknowledging where your boundaries lie and reminding you that you're respected and appreciated.