Increase amps with set voltage?
150&nbsp;Ah is a capacity rating, not a current rating. You may well find your existing battery can already deliver 150 amps.

If you want to increase the capacity from 50&nbsp;Ah to 150&nbsp;Ah, then you would need to put two more 50&nbsp;Ah batteries in parallel with the first.

This would also increase by a factor of three whatever the current rating of your first battery was, assuming they were identical.

In order to find out what the current rating of your first battery is, you would need to look at the specifications. They normally give a C rating, which can be interpreted roughly as 'hours to fully discharge the battery'. Capacity falls as the output current increases, hence the qualifier 'roughly'.

A C rating of 1 means that the current rating is numerically equal to the capacity rating, which it looks like you are assuming. However, few batteries have a maximum C of 1. 

There are different ratings for current. The two most frequently met are the maximum current that can be drawn safely, and the current for which the capacity rating is valid. With a car battery for instance, the maximum C is often around 10, for cranking the engine. However the maximum capacity C is often 0.1 or even 0.05, requiring a 10 or 20 hour discharge to obtain the stated Ah capacity.