I'm an Australian passport holder going to Thailand for 3 months in that 3 months I'll travel into Lao and then back to Thailand then later into Cambodia departing from phuket After 3 months. So what's the best visa to get or can I get 30 day visas at border crossing. 

If you have not previously traveled to Thailand (or only minimally some time ago) Thai immigration should be fine with you just using visa exempt entries. Each visa exempt entry gives you 30 days. If necessary, this can be extended by a further 30 days at an immigration office inside Thailand on payment of 1,900 baht.

 

There is just one annoying complication, not with immigration but the airline. When you fly to Thailand, the airline is likely to want to see either a visa or an onward flight ticket out of Thailand (one three months out may not qualify, depending on the check in staff). For this reason, it is probably prudent to get a single entry tourist visa to cover you for your initial entry, but visa exempt entries thereafter.

You could be denied boarding your flight to here without a visa and no ticket out of the country within 30 days of arrival.

You could get the two 30 day visa exempt entries (they are not visas) at a land border crossing but they will be the only 2 you can get this year without a problem.