Since it is 3HO, be aware that they are a sect of Sikhism that mixes with kundalini yoga, and is not typical of normal Sikhism.
So as long as you societally 'pass,' you'll be fine!
Bring a hankerchief so that when you enter the Gurdwara, the place where their Scripture (the Guru Granth) is, you'll have covered your head by then. The temple (or in some 3HO scenarios, yoga studio with Gurdwara services) may have some hankerchiefs too for you if you do not have any.
Women will generally wear a scarf, or a kerchief. Men wear kerchiefs. Both sexes who are 'baptised' (taken Amrit, which means that they take vows of Sikhism seriously) will often wear turbans to keep their hair neat.
Make sure you bring a donation, like a five dollar bill!
When you come, one can show respect by first coming towards Sri Guru Granth Sahib (the holy book), and then after praying, bow down in respect. Then you can give a donation at that front. Then, there may be a person beside who has fruit or a spongy substance [it's flour, butter, and sugar mixed together] (these are called prashad; it is food blessed by God for people who come to the Gurdwara). Kneel down and receive it with your left hand.
You may proceed to sit down - generally, men sit on one side, and women sit on the other side, so choose your gender side, lol!
You can partake of the prashad while sitting down. NEVER point your feet at the Holy Book, since it is a sign of disrespect. You can sit cross-legged. When leaving the sanctuary, make sure to bow down first at the Book before walking out.
You can leave whenever you like, but make sure you ask for 'langar' (kitchen), and they can sit you down so you can have some free vegetarian food before you go.
If all of this confuses you, just go there and enjoy. Know that the presence of God is in the Gurbani, or devotional prayers and chants.
Good luck, and Satnam!